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THERISEOFThecityofTeotihuacán,whichlayabout50kilometersnortheastofmodern-dayMexicoCity,beganitsgrowthby200-100B.C.Atitsheight,betweenaboutA.D.150and700,itprobablyhadapopulationofmorethan125,000peopleandcoveredatleast20squarekilometers.Ithadover2,000apartmentcomplexes,agreatmarket,alargenumberofindustrialworkshops,anadministrativecenter,anumberofmassivereligiousedifices,andaregulargridpatternofstreetsandbuildings.Clearly,muchnningandcentralcontrolwereinvolvedintheexpansionandorderingofthisgreatmetropolis.Moreover,thecityhadeconomicandperhapsreligiouscontactswithmostpartsofMesoamerica(modernCentralAmericaandMexico).Howdidthistremendousdevelopmenttakece,andwhydidithappenintheTeotihuacánValley?AmongthemainfactorsareTeotihuacán’sgeographiclocationonanaturaltraderoutetothesouthandeastoftheValleyofMexico,theobsidianresourcesintheTeotihuacánValleyitself,andthevalley’spotentialforextensiveirrigation.Theexactroleofotherfactorsismuchmoredifficulttopinpoint―forinstance,Teotihuacán’sreligioussignificanceasashrine,thehistoricalsituationinandaroundtheValleyofMexicotowardtheendofthefirstmillenniumB.C.,theingenuityandforesightednessofTeotihuacán’selite,and,finally,theimpactofnaturaldisasters,suchasthevolcaniceruptionsofthelatefirstmillenniumB.C.ThislastfactorisatleastcircumstantiallyimplicatedinTeotihuacán’srise.Priorto200B.C.,anumberofrelativelysmallcenterscoexistedinandneartheValleyofMexico.Aroundthistime,thelargestofthesecenters,Cuicuilco,wasseriouslyaffectedbyavolcaniceruption,withmuchofitsagriculturallandcoveredbylava.WithCuicuilcoeliminatedasapotentialrival,anyoneofanumberofrelativelymodesttownsmighthaveemergedasaleadingeconomicandpoliticalpowerinCentralMexico.Thearchaeologicalevidenceclearlyindicates,though,thatTeotiluacanwasthecenterthatdidariseasthepredominantintheareabythefirstcenturyA.D.ItseemslikelythatTeotihuacán’snaturalresources―alongwiththecityelite’sabilitytorecognizetheirpotential―gavethecityacompetitiveedgeoveritsneighbors,Thevalley,likemanyothercesinMexicanandGuatemalanhighlands,wasrichinobsidian.Thehardvolcanicstonewasaresourcethathadbeeningreatdemandformanyyears,atleastsincetheriseoftheOlmecs(apeoplewhoflourishedbetween1200and400B.C.),anditapparentlyhadasecuremarket.Moreover,recentresearchonobsidiantoolsfoundatOlmecssiteshasshownthatsomeoftheobsidianobtainedbytheOlmecsoriginatednearTeotihuacán.Teotihuacánobsidianmusthavebeenrecognizedasavaluablecommodityformanycenturiesbeforethegreatcityarose.Long-distancetradeinobsidianprobablygavetheeliteresidentsofTeotihuacánaccesstoawidevarietyofexoticgood,aswellasarelativelyprosperouslife.SuchsuccessmayhaveattractedimmigrantstoTeotihuacán.Inaddition,Teotihuacán’selitemayhaveconsciouslyattemptedtoattractnewinhabitants.Itisalsoprobablethatasearlyas200B.C.Teotihuacánmayhaveachievedsomereligioussignificanceanditsshrine(orshrines)mayhaveservedasanadditionalpopulationmagnet.Finally,thegrowingpopulationwasprobablyfedbyincreasingthenumberandsizeofirrigatedfields.ThepictureofTeotihuacánthatemergesisaclassicpictureofpositivefeedbackamongobsidianminingandworking,trade,populationgrowth,irrigation,andreligioustourism.Thethrivingobsidianoperation,forexample,wouldnecessitatemoreminers,additionalmanufacturersofobsidiantools,andadditionaltraderstocarrythegoodstonewmarkets.Allthisledtoincreasedwealth,whichinturnwouldattractmoreimmigrantstoTeotihuacán.Thegrowingpoweroftheelite,whocontrolledtheeconomy,wouldgivethemthemeanstophysicallycoercepeopletomovetoTeotihuacánandserveasadditionstothelabor.Moreirrigationworkswouldhavetobebuilttofeedthegrowingpopulation,andthisresultedinmorepowerandwealthfortheelite.Paragraph1ThecityofTeotihuacán,whichlayabout50kilometersnortheastofmodern-DayMexicoCity,beganitsgrowthby200-100B.C.Atitsheight,betweenaboutA.D.150and700,itprobablyhadapopulationofmorethan125,000peopleandcoveredatleast20squarekilometers.Ithadover2,000apartmentcomplexes,agreatmarket,alargenumberofindustrialworkshops,anadministrativecenter,anumberofmassivereligiousedifices,andaregulargridpatternofstreetsandbuildings.Clearly,muchnningandcentralcontrolwereinvolvedintheexpansionandorderingofhisgreatmetropolis.Moreover,thecityhadeconomicandperhapsreligiouscontactswithmostpartsofMesoamerica(modernCentralAmericaandMexico).ThewordmassiveinthepassageisclosestinmeaningverycarefullyInparagraph1,eachofthefollowingismentionedasafeatureofthecityofTeotihuacánbetweenA.D.150and700EXCEPTregularlyarrangedseveraladministrativecentersspreadacrossthemanymanufacturingapartmentParagraph2Howdidthistremendousdevelopmenttakece,andwhydidithappenintheTeotihuacánValley?AmongthemainfactorsareTeotihuacán’sgeographiclocationonanaturaltraderoutetothesouthandeastoftheValleyofMexico,theobsidianresourcesintheTeotihuacánValleyitself,andthevalley’spotentialforextensiveirrigation.Theexactroleofotherfactorsismuchmoredifficulttopinpoint―forinstance,Teotihuacán’sreligioussignificanceasashrine,thehistoricalsituationinandaroundtheValleyofMexicotowardtheendofthefirstmillenniumB.C.,theingenuityandforesightednessofTeotihuacán’selite,and,finally,theimpactofnaturaldisasters,suchasthevolcaniceruptionsofthelatefirstmillenniumB.C.ThewordpinpointinthepassageisclosestinmeaningidentifymakeanargumentThewordingenuityinthepassageisclosestinmeaningWhichofthefollowingisNOTmentionedinparagraph2asamainfactorinThedevelopmentofThepresenceofobsidianintheTeotihuacánThepotentialforextensiveirrigationofTeotihuacánValleyAlongperiodofvolcanicinactivityintheTeotihuacánTeotihuacán’slocationonanaturaltradeParagraph2Howdidthistremendousdevelopmenttakece,andwhydidithappenintheTeotihuacánValley?AmongthemainfactorsareTeotihuacán’sgeographiclocationonanaturaltraderoutetothesouthandeastoftheValleyofMexico,theobsidianresourcesintheTeotihuacánValleyitself,andthevalley’spotentialforextensiveirrigation.Theexactroleofotherfactorsismuchmoredifficulttopinpoint―forinstance,Teotihuacán’sreligioussignificanceasashrine,thehistoricalsituationinandaroundtheValleyofMexicotowardtheendofthefirstmillenniumB.C.,theingenuityandforesightednessofTeotihuacán’selite,and,finally,theimpactofnaturaldisasters,suchasthevolcaniceruptionsofthelatefirstmillenniumB.C.Paragraph3ThislastfactorisatleastcircumstantiallyimplicatedinTeotihuacán’srise.Priorto200B.C.,anumberofrelativelysmallcenterscoexistedinandneartheValleyofMexico.Aroundthistime,thelargestofthesecenters,Cuicuilco,wasseriouslyaffectedbyavolcaniceruption,withmuchofitsagriculturallandcoveredbylava.WithCuicuilcoeliminatedasapotentialrival,anyoneofanumberofrelativelymodesttownsmighthaveemergedasaleadingeconomicandpoliticalpowerinCentralMexico.Thearchaeologicalevidenceclearlyindicates,though,thatTeotiluacanwasthecenterthatdidariseasthepredominantintheareabythefirstcenturyA.D.Whichofthefollowingcanbeinferredfromparagraphs2and3abouttheVolcaniceruptionsofthelatefirstmillenniumB.C.?TheyweremorefrequentthanhistoriansonceTheymayhavedonemoredamagetoTeotihuacánthantoneighboringTheymayhaveyedamajorroleintheriseofTheyincreasedtheneedforextensiveirrigationintheTeotihuacánWhatcanbeinferredfromparagraph3aboutCuicuilcopriorto200ItwasafairlysmallcityuntilthatItwaslocatedoutsidetheValleyofItemergedrapidlyasaneconomicalandpoliticalItseconomyreliedheavilyonThewordpredominantinthepassageisclosestinmeaningmostmostParagraph4ItseemslikelythatTeotihuacán’snaturalresources―alongwiththecityelite’sabilitytorecognizetheirpotential―gavethecityacompetitiveedgeoveritsneighbors,Thevalley,likemanyothercesinandGuatemalanhighlands,wasrichinobsidian.Thehardvolcanicstonewasaresourcethathadbeeningreatdemandformanyyears,atleastsincetheriseoftheOlmecs(apeoplewhoflourishedbetween1200and400B.C.),anditapparentlyhadasecuremarket.Moreover,recentresearchonobsidiantoolsfoundatOlmecssiteshasshownthatsomeoftheobsidianobtainedbytheOlmecsoriginatednearTeotihuacán.Teotihuacánobsidianmusthavebeenrecognizedasavaluablecommodityformanycenturiesbeforethegreatcityarose.WhichofthefollowingallowedTeotihuacántohave“acompetitiveedgeoveritsAwell-exploitedandreadilyavailableThepresenceofahighlystableeliteKnowledgederiveddirectlyfromtheOlmecsabouttheartoftoolScarcenaturalresourcesinnearbyareassuchasthoselocatedinwhatarenowtheGuatemalanandMexicanAccordingtoparagraph4,whathasrecentresearchonobsidiantoolsfoundatOlmecsitesObsidian’svaluewasunderstoodonlywhenTeotihuacánbecameanimportantTheresidentsofTeotihuacánweresophisticatedTheresidentsofTeotihuacántradedobsidianwiththeOlmecsasearlyas400SomeoftheobsidianusedbytheOlmecscamefromtheareaaroundParagraph5Long-distancetradeinobsidianprobablygavetheeliteresidentsofTeotihuacánaccesstoawidevarietyofexoticgood,aswellasarelativelyprosperouslife.SuchsuccessmayhaveattractedimmigrantstoTeotihuacán.Inaddition,Teotihuacán’selitemayhaveconsciouslyattemptedtoattractnewinhabitants.Itisalsoprobablethatasearlyas200B.C.Teotihuacánmayhaveachievedsomereligioussignificanceanditsshrine(orshrines)mayhaveservedasanadditionalpopulationmagnet.Finally,thegrowingpopulationwasprobablyfedbyincreasingthenumberandsizeofirrigatedfields.SelecttheTWOanswerchoicesthatarementionedinparagraph5asbeingfeaturesofTeotihuacánthatmayhaveattractedimmigrantstothecity.Toreceivecredit,youmustselectTWOanswers.TheprosperityofthePlentyofavailableOpportunitiesforwell-paidagriculturalThepresenceofoneormorereligiousParagraph6ThepictureofTeotihuacánthatemergesisaclassicpictureofpositivefeedbackamongobsidianminingandworking,trade,populationgrowth,irrigation,andreligioustourism.Thethrivingobsidianoperation,forexample,wouldnecessitatemoreminers,additionalmanufacturersofobsidiantools,andadditionaltraderstocarrythegoodstonewmarkets.Allthisledtoincreasedwealth,whichinturnwouldattractmoreimmigrantstoTeotihuacán.Thegrowingpoweroftheelite,whocontrolledtheeconomy,wouldgivethemthemeanstophysicallycoercepeopletomovetoTeotihuacánandserveasadditionstothelabor.Moreirrigationworkswouldhavetobebuilttofeedthegrowingpopulation,andthisresultedinmorepowerandwealthfortheelite.Inparagraph6,theauthordiscussesthe“Thethrivingobsidianoperation”inorderexinwhymanufacturingwasthemainindustryofgiveanexampleofanindustrythattookverylittletimetodevelopinIllustratehowseveralfactorsinfluencedeachothertomakeTeotihuacánapowerfulandwealthyexinhowasuccessfulindustrycanbeasourceofwealthandasource atthesameThecityofTeotihuacán,whichlayabout50kilometersnortheastofmodern-DayMexicoCity,beganitsgrowthby200-100B.C.Atitsheight,betweenaboutA.D.150and700,itprobablyhadapopulationofmorethan125,000peopleandcoveredatleast20squarekilometers.■Ithadover2,000apartmentcomplexes,agreatmarket,alargenumberofindustrialworkshops,anadministrativecenter,anumberofmassivereligiousedifices,andaregulargridpatternofstreetsandbuildings.■Clearly,muchnningandcentralcontrolwereinvolvedintheexpansionandorderingofthisgreatmetropolis.■Moreover,thecityhadeconomicandperhapsreligiouscontactswithmostpartsofMesoamerica(modernCentralAmericaandMexico).■Infact,artifactsandpotteryfromTeotihuacánhavebeendiscoveredinsitesasfarawayastheMayanlowlands,theGuatemalanhighlands,northernMexico,andtheGulfCoastofMexico.TeotihuacánwasahighlydevelopedcityinMesoamericathatreacheditspeakbetweenaboutA.D.150andThenumberandsophisticationofthearchitectural,administrative,commercial,andreligiousfeaturesofTeotihuacanindicatetheexistenceofcentralizednningandcontrol.Teotihuacánmayhavedevelopeditsownspecificlocalreligionasaresultoftheculturaladvancespossiblebythecity’sgreatAsaresultofitslargenumberofreligiousshrines,bythefirstcenturyA.D.,Teotihuacan ethemostresources,irrigationpotential,inligentelite,andthemisfortuneofrivalInmanyimportantareas,fromtheobsidianindustrytoreligioustourism,Teotihuacán’ssuccessprosperitytypifiedtheclassicpositivefeedbackAlthoughmanyimmigrantssettledinTeotihuacánbetweenA.D.150and700,theincreasingthreatofcoercedlabordiscouragedfurthersettlementandlimitedTeotihuacán’spopulationgrowth.参考答案：1. 2. 3. 4. 5. 6. 7. 9. 10. 11.○1 12. 13.○

特坎的崛于公元前到年前的特坎城位于现在的墨西哥城东北约公里处。在鼎盛时期，也就是大公元150到一个行政、数量庞大的场所还有规则的街道建筑网络。显然，这座伟大的都市的管理和扩张时经过了精心的规划和集中管理的。甚至特坎城与洲的大部分都保持着经济和的联系。这惊人的发展是如何完成的呢，另外它为什么会发生在奥特峡谷呢？其中最主要的原因就是奥特地处墨西哥峡谷南部和东部的自然形成的线路中，奥特峡谷本身拥有的黑曜石资源，还有奥特峡谷大面积灌溉的潜能。而其他的因素的作用则很难表述清楚——例如，奥特作为的重要地位，在公元前一千年后期墨西哥峡谷及其周围地区的历史情况，奥特精英们的机智和深谋远虑，以及自然的冲击，比如在公元前一千年后期的火山喷发。和谐共存着。就在这时其中最大的中心，CuicuilcoCuicuilco长距离的黑曜岩可能就使得特坎的精英们有机会得到外来的商品和繁荣的生活，这种成功可能会吸引到特EXTINCTIONOFTHEPaleontologistshavearguedforalongtimethatthedemiseofthedinosaurswascausedbyclimaticaltionsassociatedwithslowchangesinthepositionsofcontinentsandseasresultingfromtetectonics.OffandonthroughouttheCretaceous(thelastperiodoftheMesozoicera,duringwhichdinosaursflourished),largeshallowseascoveredextensiveareasofthecontinents.Datafromdiversesources,includinggeochemicalevidencepinseafloorsediments,indicatethattheLateCretaceousclimatewasmilderthantoday’s.Thedayswerenottoohot,northenightstoocold.Thesummerswerenottoowarm,northewinterstoofrigid.Theshallowseasonthecontinentsprobablybufferedthetemperatureofthenearbyair,keeitrelativelyconstant.AttheendoftheCretaceous,thegeologicalrecordshowsthattheseseawaysretreatedfromthecontinentsbackintothemajoroceanbasins.Nooneknowswhy.Overaperiodofabout100,000years,whiletheseaspulledback,climatesaroundtheworldbecamedramaticallymoreextreme:warmerdays,coolernights;hottersummers,colderwinters.Perhapsdinosaurscouldnottoleratetheseextremetemperaturechangesandbecameextinct.Iftrue,though,whydidcold-bloodedanimalssuchassnakes,lizards,turtles,andcrocodilessurvivethezingwintersandtorridsummers?Theseanimalsareatthemercyoftheclimatetomaintainalivablebodytemperature.It’shardtounderstandwhytheywouldnotbeaffected,whereasdinosaurswerelefttoocrippledtocope,especiallyif,assomescientistsbelieve,dinosaurswerewarm-blooded.CriticsalsopointoutthattheshallowseawayshadretreatedfromandadvancedonthecontinentsnumeroustimesduringtheMesozoic,sowhydidthedinosaurssurvivetheclimaticchangesassociatedwiththeearlierfluctuationsbutnotwiththisone?Althoughinitiallyappealing,thehypothesisofasimpleclimaticchangerelatedtosealevelsisinsufficienttoexinalltheDissatisfactionwithconventionalexnationsfordinosaurextinctionsledtoasurprisingobservationthat,inturn,hassuggestedanewhypothesis.ManyntsandanimalsdisappearabruptlyfromthefossilrecordasonemovesfromlayersofrockingtheendoftheCretaceousupintorocksrepresentingthebeginningoftheCenozoic(theeraaftertheMesozoic).BetweenthelastlayerofCretaceousrockandthefirstlayerofCenozoicrock,thereisoftenathinlayerofclay.Scientistsfeltthattheycouldgetanideaofhowlongtheextinctionstookbydetermininghowlongittooktodepositthisonecentimeterofclayandtheythoughttheycoulddeterminethetimeittooktodeposittheclaybydeterminingtheamountoftheelementiridium(lr)itcontained.IthasnotbeencommonatEarth’ssincetheverybeginningofthenet’shistory.Becauseitusuallyexistsinametallicstate,itwaspreferentiallyincorporatedinEarth’scoreasthenetcooledandconsolidated.lrisfoundinhighconcentrationsinsomemeteorites,inwhichthesolarsystem’soriginalchemicalcompositionisp.Eventoday,microscopicmeteoritescontinuallybombardEarth,fallingonbothlandandsea.BymeasuringhowmanyofthesemeteoritesfalltoEarthoveragivenperiodoftime,scientistscanestimatehowlongitmighthavetakentodeposittheobservedamountoflrintheboundaryclay.Thesecalculationssuggestthataperiodofaboutonemillionyearswouldhavebeenrequired.However,otherreliableevidencesuggeststhatthedepositionoftheboundaryclaycouldnothavetakenonemillionyears.Sotheunusuallyhighconcentrationoflrseemstorequireaspecialexnation.Inviewofthesefacts,scientistshypothesizedthatasinglelargeasteroid,about10to15kilometersacross,collidedwithEarth,andtheresultingfalloutcreatedtheboundaryclay.Theircalculationsshowthattheimpactkickedupadustcloudthatcutoffsunlightforseveralmonths,inhibitingphotosynthesisinnts;decreasedsurfacetemperaturesoncontinentstobelowzing;causedextremeepisodesofacidrain;andsignificantlyraisedlong-termglobaltemperaturesthroughthegreenhouseeffect.Thisdisruptionoffoodchainandclimatewouldhaveeradicatedthedinosaursandotherorganismsinlessthanfiftyyears.Paragraph1Paleontologistshavearguedforalongtimethatthedemiseofthedinosaurswascausedbyclimaticaltionsassociatedwithslowchangesinthepositionsofcontinentsandseasresultingfromtetectonics.OffandonthroughouttheCretaceous(thelastperiodoftheMesozoicera,duringwhichdinosaursflourished),largeshallowseascoveredextensiveareasofthecontinents.Datafromdiversesources,includinggeochemicalevidencepinseafloorsediments,indicatethattheLateCretaceousclimatewasmilderthantoday’s.Thedayswerenottoohot,northenightstoocold.Thesummerswerenottoowarm,northewinterstoofrigid.Theshallowseasonthecontinentsprobablybufferedthetemperatureofthenearbyair,keeitrelativelyconstant.Accordingtoparagraph1,whichofthefollowingistrueoftheLateCretaceousSummerswereverywarmandwinterswereveryShallowseasonthecontinentscausedfrequenttemperatureTheclimatewasverysimilartotoday’sTheclimatedidnotchangedramaticallyfromseasontoParagraph2AttheendoftheCretaceous,thegeologicalrecordshowsthattheseseawaysretreatedfromthecontinentsbackintothemajoroceanbasins.Nooneknowswhy.Overaperiodofabout100,000years,whiletheseaspulledback,climatesaroundtheworldbecamedramaticallymoreextreme:warmerdays,coolernights;hottersummers,colderwinters.PerhapsdinosaurscouldnottoleratetheseextremetemperaturechangesandbecameWhichofthefollowingreasonsissuggestedinparagraph2fortheextinctionoftheChangesinthelengthsofthedaysandnightsduringthelateCretaceousDroughtscausedbythemovementofseawaysbackintotheThechangefrommildtosevereclimatesduringtheLateCretaceousAnextremedecreaseintheaverageyearlytemperatureover10,oooParagraph3Iftrue,though,whydidcold-bloodedanimalssuchassnakes,lizards,turtles,andcrocodilessurvivethezingwintersandtorridsummers?Theseanimalsareatthemercyoftheclimatetomaintainalivablebodytemperature.It’shardtounderstandwhytheywouldnotbeaffected,whereasdinosaurswerelefttoocrippledtocope,especiallyif,assomescientistsbelieve,dinosaurswerewarm-blooded.CriticsalsopointoutthattheshallowseawayshadretreatedfromandadvancedonthecontinentsnumeroustimesduringtheMesozoic,sowhydidthedinosaurssurvivetheclimaticchangesassociatedwiththeearlierfluctuationsbutnotwiththisone?Althoughinitiallyappealing,thehypothesisofasimpleclimaticchangerelatedtosealevelsisinsufficienttoexinalltheWhydoestheauthormentionthesurvivalof“snakes,lizards,turtles,andcrocodiles”inparagraphToarguethatdinosaursmay eextinctbecausetheywerenotcold-bloodedToquestiontheadequacyofthehypothesisthatclimaticchangerelatedtosealevelscausedtheextinctionofthedinosaursTopresentexamplesofanimalsthatcouldmaintainalivablebodytemperaturemoreeasilythanTosupportahypothesisthattheseanimalswerenotassensitivetoclimatechangesintheCretaceousperiodastheyaretodayThewordcopeinthepassageisclosestinmeaningAccordingtoparagraph3,whichofthefollowingistrueofchangesinclimatebeforetheCretaceousperiodandtheeffectofthesechangesondinosaurs?ClimatechangesassociatedwiththemovementofseawaysbeforetheCretaceousperioddidnotdinosaurs eChangesinclimatebeforetheCretaceousperiodcausedseverefluctuationsinsealevel,resultingintheextinctionofthedinosaurs.FrequentchangesinclimatebeforetheCretaceousperiodmadedinosaursbetterabletomaintainalivablebodytemperature.BeforetheCretaceousperiodtherewerefewchangesinclimate,anddinosaursThewordfluctuationsinthepassageisclosestinmeaningParagraph4Dissatisfactionwithconventionalexnationsfordinosaurextinctionsledtoasurprisingobservationthat,inturn,hassuggestedanewhypothesis.Manyntsandanimalsdisappearabruptlyfromthefossilrecordasonemovesfromlayersofrock ingtheendoftheCretaceousupintorocksthebeginningoftheCenozoic(theeraaftertheMesozoic).BetweenthelastlayerofCretaceousrockandthefirstlayerofCenozoicrock,thereisoftenathinlayerofclay.Scientistsfeltthattheycouldgetanideaofhowlongtheextinctionstookbydetermininghowlongittooktodepositthisonecentimeterofclayandtheythoughttheycoulddeterminethetimeittooktodeposittheclaybydeterminingtheamountoftheelementiridium(lr)itcontained.Whichofthesentencesbelowbestexpressestheessentialinformationinthehighlightedsentenceinthepassage?Incorrectchoiceschangethemeaninginimportantwaysorleaveoutessentialinformation.ThefossilrecordsuggeststhattherewasanabruptextinctionofmanyntsandanimalsattheendofMesozoicFewfossilsoftheMesozoicerahavesurvivedintherocksthatmarktheendoftheFossilsfromtheCretaceousperiodoftheMesozoicuptothebeginningoftheCenozoicerahavebeenremovedfromthelayersofrockthatsurroundedthem.ntsandanimalsfromtheMesozoicerawereunabletosurviveintheCenozoicParagraph4Dissatisfactionwithconventionalexnationsfordinosaurextinctionsledtoasurprisingobservationthat,inturn,hassuggestedanewhypothesis.ManyntsandanimalsdisappearabruptlyfromthefossilrecordasonemovesfromlayersofrockingtheendoftheCretaceousupintorocksrepresentingthebeginningoftheCenozoic(theeraaftertheMesozoic).BetweenthelastlayerofCretaceousrockandthefirstlayerofCenozoicrock,thereisoftenathinlayerofclay.Scientistsfeltthattheycouldgetanideaofhowlongtheextinctionstookbydetermininghowlongittooktodepositthisonecentimeterofclayandtheythoughttheycoulddeterminethetimeittooktodeposittheclaybydeterminingtheamountoftheelementiridium(lr)itcontained.Inparagraph4,allthefollowingquestionsareansweredWhyistherealayerofclaybetweentherocksoftheCretaceousandWhywerescientistsinterestedindetermininghowlongittooktodepositthelayerofclayattheendoftheWhatwastheeffectofthesurprisingobservationscientistsWhydidscientistswantmoreinformationaboutthedinosaurextinctionsattheendoftheParagraph5lrhasnotbeencommonatEarth’ssincetheverybeginningofthenet’shistory.Becauseitusuallyexistsinametallicstate,itwaspreferentiallyincorporatedinEarth’scoreasthenetcooledandconsolidated.lrisfoundinhighconcentrationsinsomemeteorites,inwhichthesolarsystem’soriginalchemicalcompositionisp.Eventoday,microscopicmeteoritescontinuallybombardEarth,fallingonbothlandandsea.BymeasuringhowmanyofthesemeteoritesfalltoEarthoveragivenperiodoftime,scientistscanestimatehowlongitmighthavetakentodeposittheobservedamountoflrintheboundaryclay.Thesecalculationssuggestthataperiodofaboutonemillionyearswouldhavebeenrequired.However,otherreliableevidencesuggeststhatthedepositionoftheboundaryclaycouldnothavetakenonemillionyears.Sotheunusuallyhighconcentrationoflrseemstorequireaspecialexnation.ThewordbombardinthepassageisclosestinmeaningParagraph5impliesthataspecialexnationoflrintheboundaryclayisneededthelrinmicroscopicmeteoritesreachingEarthduringtheCretaceousperiodwouldhavebeenintoEarth’sthelrintheboundaryclaywasdepositedmuchmorethanamillionyearstheconcentrationoflrintheboundaryclayishigherthaninmicroscopictheamountoflrintheboundaryclayistoogreattohavecomefrommicroscopicmeteoritesduringthetimetheboundaryclaywasdepositedParagraph6Inviewofthesefacts,scientistshypothesizedthatasinglelargeasteroid,about10to15kilometersacross,collidedwithEarth,andtheresultingfalloutcreatedtheboundaryclay.Theircalculationsshowthattheimpactkickedupadustcloudthatcutoffsunlightforseveralmonths,inhibitingphotosynthesisinnts;decreasedsurfacetemperaturesoncontinentstobelowzing;causedextremeepisodesofacidrain;andsignificantlyraisedlong-termglobaltemperaturesthroughthegreenhouseeffect.Thisdisruptionoffoodchainandclimatewouldhaveeradicatedthedinosaursandotherorganismsinlessthanfiftyyears.TheworddisruptioninthepassageisclosestinmeaningParagraph6mentionsallofthefollowingeffectsofthehypothesizedasteroidcollisionalargedustcloudthatblockedanimmediatedropinthesurfacetemperaturesoftheanextremedecreaseinrainfallonthealong-termincreaseinglobalParagraph5IrhasnotbeencommonatEarth’ssincetheverybeginningofthenet’shistory.Becauseitusuallyexistsinametallicstate,itwaspreferentiallyincorporatedinEarth’scoreasthenetcooledandconsolidated.lrisfoundinhighconcentrationsinsomemeteorites,inwhichthesolarsystem’soriginalchemicalcompositionisp.Eventoday,microscopicmeteoritescontinuallybombardEarth,fallingonbothlandandsea.BymeasuringhowmanyofthesemeteoritesfalltoEarthoveragivenperiodoftime,scientistscanestimatethataperiodofaboutonemillionyearswouldhavebeenrequired.■However,otherreliableevidencesuggeststhedepositionoftheboundaryclaycouldnothavetakenonemillionyears.■Sotheunusuallyhighconcentrationoflrseemstorequireaspecialexnation.■Consequently,theideathatthelrintheboundaryclaycamefrommicroscopicmeteoritescannotbeaccepted.ForalongtimescientistshavearguedthattheextinctionofThedinosaurswasrelatedtoclimateAsimpleclimatechangedoesnotexinsomeimportantdatarelatedtotheextinctionofthedinosaurstheendoftheTheretreatoftheseawaysattheendoftheCretaceoushasnotbeenfullyexTheabruptnessofextinctionsattheendoftheCretaceousandthehighconcentrationoflrfound