托福阅读模拟题：宇宙元素

TheelementsotherthanhydrogenandheliumexistInsuchsmallquantitiesthatitisaccuratetosaythattheuniversesomewhatmorethan25percentheliumbyweightandsomewhatlessthan25percenthydrogen.Astronomershavemeasuredtheabundanceofheliumthroughoutourgalaxyandinothergalaxiesaswell.HeliumhasbeenfoundInoldstars,inrelativelyyoungones,ininterstellargas,andinthedistantobjectsknownasquasars.Heliumnucleihavealsobeenfoundtobeconstituentsofcosmicraysthatfallontheearth(cosmic"rays"arenotreallyaformofradiation;theyconsistofrapidlymovingparticlesofnumerousdifferentkinds).Itdoesn'tseemtomakeverymuchdifferencewheretheheliumisfound.Itsrelativeabundanceneverseemstovarymuch.Insomeplaces,theremaybeslightlymoreofit;Inothers,slightlyless,buttheratioofheliumtohydrogennucleialwaysremainsaboutthesame.Heliumiscreatedinstars.Infact,nuclearreactionsthatconverthydrogentoheliumareresponsibleformostoftheenergythatstarsproduce.However,theamountofheliumthatcouldhavebeenproducedinthismannercanbecalculated,anditturnsouttobenomorethanafewpercent.Theuniversehasnotexistedlongenoughforthisfiguretohesignificantlygreater.Consequently,iftheuniverseissomewhatmorethan25percentheliumnow,thenitmusthavebeenabout25percentheliumatatimenearthebeginning..However,whentheuniversewaslessthanoneminuteold,noheliumcouldhaveexisted.Calculationsindicatethatbeforethistimetemperaturesweretoohighandparticlesofmatterweremovingaroundmuchtoorapidly.Itwasonlyaftertheone-minutepointthatheliumcouldexist.Bythistime,theuniversehadcooledsufficientlythatneutronsandprotonscouldsticktogether.Butthenuclearreactionsthatledtotheformationofheliumwentonforonlyarelativelyshorttime.Bythetimetheuniversewasafewminutesold,heliumproductionhadeffectivelyceased.12.whatdoesthepassagemainlyexplain?HowstarsproduceenergyThedifferencebetweenheliumandhydrogen

Whenmostoftheheliumintheuniversewasformed(D)Whyhydrogenisabundant13.Accordingtothepassage,heliumis(A)thesecond-mostabundantelementintheuniverse(B)difficulttodetect(C)theoldestelementintheuniverse(D)themostprevalentelementinquasars14.Theword"constituents"inline7isclosestinmeaningto(A)relatives(B)causes(C)componentstargets15.Whydoestheauthormention"cosmicrays't'inline7?Aspartofalistofthingscontaininghelium(B)Asanexampleofanunsolvedastronomicalpuzzle(C)Toexplainhowtheuniversebegan(D)Toexplaintheabundanceofhydrogenintheuniverse16.Theword"vary"inline10isclosestillmeaningto(A)mean(B)stretch(C)change(D)include17.Thecreationofheliumwithinstarscannotbemeasuredproducesenergyproduceshydrogenasaby-product(D)causesheliumtobemuchmoreabundantInoldstarsthanInyoungstar:Theword"calculated"inline15isclosestinmeaningtoignoredconvertedincreaseddeterminedMostoftheheliumintheuniversewasformedininterstellarspaceinaveryshorttime