2025年ACT《科学推理》历年真题

2025年ACT《科学推理》历年真题考试时间：______分钟总分：______分姓名：______模拟试卷第一部分1.Aresearcherinvestigatedtheeffectoftwodifferentfertilizers(F1andF2)onplantheight.Plantsweregrownincontrolledconditionswithequalamountsofsunlightandwater.GroupAreceivedF1,andGroupBreceivedF2.Attheendofthestudy,theaverageheights(incentimeters)of30plantsineachgroupwererecorded.TheresultsshowedthattheaverageheightofplantsinGroupAwas45cm,whiletheaverageheightofplantsinGroupBwas50cm.Whichofthefollowingconclusionscanbemoststronglydrawnfromthisstudy?(A)F2ismoreeffectivethanF1inpromotingplantheightundertheseconditions.(B)Thedifferenceinplantheightbetweenthegroupsisduesolelytothetypeoffertilizerused.(C)Thestudydesigneffectivelycontrolledforvariablesotherthanthetypeoffertilizer.(D)PlantsinGroupBreceivedmoresunlightthanplantsinGroupA.2.Abiologistobservedthatinacertainpopulationofmice,thealleleforbrownfur(B)isdominantoverthealleleforwhitefur(b).Inageneticcrossbetweentwoheterozygousbrownmice(BbxBb),theexpectedgenotypicratiooftheiroffspringis:(A)100%BB(B)50%Bband50%bb(C)25%BB,50%Bb,and25%bb(D)75%Bband25%bb3.Thehalf-lifeofaradioactiveisotopeisthetimerequiredforhalfofasampletodecay.Ifa100-gramsampleofIsotopeXhasahalf-lifeof5years,howmuchofIsotopeXwouldremainafter20years?(A)12.5grams(B)25grams(C)50grams(D)100grams4.WhichofthefollowingbestdescribestherelationshipbetweentheatmosphereandtheEarth'stemperature?(A)Theatmospherereflectssunlightbackintospace,causingtheEarthtocool.(B)TheatmosphereabsorbsallinfraredradiationemittedbytheEarth,preventinganyheatloss.(C)TheatmospheretrapssomeoftheEarth'soutgoinginfraredradiation,leadingtothegreenhouseeffectandwarmingtheplanet.(D)Theatmosphereprimarilytransfersheatfromtheequatortothepolesthroughconvectioncurrentsalone.5.Acaracceleratesuniformlyfromrest.After6seconds,itsvelocityreaches30meterspersecond.Whatisthecar'sacceleration?(A)0.33m/s²(B)5m/s²(C)180m/s²(D)1800m/s²6.Asampleofgasoccupies2.0litersatapressureof1.0atm.Ifthetemperatureremainsconstant,whatpressurewillthegasexertifitiscompressedto0.5liters?(A)0.25atm(B)0.50atm(C)2.0atm(D)4.0atm7.Whichofthefollowingisacharacteristicofachemicalchange?(A)Changeinstatefromsolidtoliquid(B)Releaseofenergyintheformoflight(C)Formationofnewsubstanceswithdifferentproperties(D)Separationofamixtureintoitscomponents8.Anobjectisthrownverticallyupwardwithaninitialvelocityof50meterspersecond.Ignoringairresistance,whatwillbeitsvelocityatthehighestpointofitstrajectory?(A)0m/s(B)25m/supward(C)50m/supward(D)50m/sdownward9.Whichofthefollowingstatementsaccuratelydescribesthewatercycle?(A)Evaporationoccursonlyinoceans.(B)Condensationalwaysresultsintheformationofprecipitation.(C)GroundwateristheprimarysourceoffreshwateronEarth.(D)Transpirationistheprocessbywhichwatermovesfromrootstoleaves.10.Astudyclaimsthatlisteningtoclassicalmusicimprovesmathematicalperformance.Participantsweredividedintotwogroups:onegrouplistenedtoclassicalmusicforanhourbeforeamathtest,whiletheotherdidnot.Thegroupthatlistenedtomusicscoredslightlyhigher,onaverage.Whichofthefollowingfactorscouldpotentiallyexplaintheobserveddifferenceinscores,suggestingtheresultmightnotbesolelyduetothemusic?(A)Participantsinthemusicgroupweregenerallymoremotivatedtoperformwell.(B)Themathtestgiventothemusicgroupwaseasierthantheonegiventothecontrolgroup.(C)Theresearchersconductingthetestforthemusicgroupweresubconsciouslyencouragingtheparticipants.(D)Thesamplesizewastoosmalltodrawadefinitiveconclusion.第二部分11.Ageologistisstudyingrocklayersinaregion.LayerAisclearlyvisibleaboveLayerB.FossilsofspeciesX,knowntohavelivedonlyinawarm,shallowmarineenvironment,arefoundexclusivelyinLayerB.LayerAcontainsfossilsofspeciesY,adaptedtoaforestenvironment.Basedonthisinformation,whichconclusionismostsupported?(A)LayerAisolderthanLayerB.(B)Theareawheretheserocklayersformedwasonceaforest.(C)Theareawheretheserocklayersformedwasonceashallowsea.(D)SpeciesXandspeciesYlivedatthesametime.12.Thediagrambelowrepresentsasimpleelectricalcircuitconsistingofabattery,aresistorR1,andaparallelcombinationoftworesistors,R2andR3.[Note:Nodiagramisprovidedasrequested]Assumethecircuithasatotalcurrentof3ampsfromthebattery.IfR1hasaresistanceof4ohmsandR2hasaresistanceof6ohms,whatisthecurrentflowingthroughR2?(A)0.33A(B)0.60A(C)1.00A(D)1.80A13.WhichofthefollowingstatementsbestexplainswhytheEarthhasseasons?(A)TheEarth'sdistancefromtheSunvariesthroughouttheyear.(B)TheSun'sintensitychangesasitorbitstheEarth.(C)ThetiltoftheEarth'srotationalaxisrelativetoitsorbitaroundtheSun.(D)TheEarth'satmosphereblockssunlightduringcertainpartsoftheyear.14.Abiologistmeasuresthepopulationgrowthofabacterialcultureoverseveralhours.Thepopulationdoublesevery30minutes.Iftheinitialpopulationis100bacteria,whatwilltheapproximatepopulationbeafter4.5hours?(A)1,600bacteria(B)6,400bacteria(C)12,800bacteria(D)51,200bacteria15.Whichofthefollowingisaprimaryfunctionofthemitochondriainaeukaryoticcell?(A)SynthesisofDNA(B)Proteinsynthesis(C)GenerationofATPthroughcellularrespiration(D)Transportofmaterialsacrossthecellmembrane16.Achemicalequationshowsthereactionbetweenhydrogengas(H₂)andoxygengas(O₂)toformwater(H₂O).If4gramsofhydrogenreactcompletely,howmanygramsofwaterwillbeproduced?(Assumecompletereactionanduseatomicmasses:H=1g/mol,O=16g/mol).(A)8grams(B)16grams(C)32grams(D)36grams17.Anexperimentteststheeffectofdifferentlightintensitiesontherateofphotosynthesisinaplant.Theresultsaremeasuredbytheamountofoxygenproduced.Whichofthefollowinggraphsbestrepresentstheexpectedrelationshipbetweenlightintensityandtherateofphotosynthesis,assumingotherfactorsareheldconstant?(A)Agraphshowingalinearincreaseinoxygenproductionwithincreasinglightintensity,reachingaplateauatveryhighintensities.(B)Agraphshowingasharpincreaseinoxygenproductionatlowlightintensities,followedbyagraduallevelingoff.(C)Agraphshowingalineardecreaseinoxygenproductionaslightintensityincreases.(D)Agraphshowingnorelationshipbetweenlightintensityandoxygenproduction.18.Whichofthefollowingstatementsisacharacteristicofthecellmembrane?(A)ItisprimarilycomposedofproteinsandDNA.(B)Itisrigidanddoesnotchangeshape.(C)Itallowscertainsubstancestopassinandoutofthecellwhileblockingothers.(D)Itcontainsthecell'sgeneticmaterial.19.Aphysiciststudiesthemotionofaprojectilelaunchedatanangle.Whichofthefollowingcomponentsofitsmotionisaffectedbyairresistance?(A)Thehorizontalcomponentofvelocity(B)Theverticalcomponentofvelocity(C)Theaccelerationduetogravity(D)Thetotalmechanicalenergyoftheprojectile20.Twoobjects,AandB,havethesamemass.ObjectAismovingatavelocityof10m/s,andObjectBisstationary.Whichstatementistrueregardingtheirmomenta?(A)ObjectAhasagreatermomentumthanObjectB.(B)ObjectBhasagreatermomentumthanObjectA.(C)Bothobjectshavethesamemomentum.(D)Themomentumofeachobjectcannotbedeterminedwithoutknowingtheirmasses.第三部分21.Aplanetorbitsastarinanearlycircularpath.Whichofthefollowingstatementsbestdescribestheforceresponsibleforkeepingtheplanetinitsorbit?(A)Thegravitationalforceexertedbythestarontheplanet.(B)Thefrictionalforcebetweentheplanetandtheinterstellarmedium.(C)Theelectromagneticforcebetweentheplanetandthestar.(D)Thecentripetalforcegeneratedbytheplanet'srotation.22.Aresearcherisstudyingtheeffectsofanewdrugonbloodpressure.Heconductsanexperimentwithtwogroupsof50patientseach.Group1receivesthedrug,andGroup2receivesaplacebo.Beforeadministeringthedrug,theaveragesystolicbloodpressureforGroup1is130mmHg,andforGroup2,itis128mmHg.Afteronemonth,theaveragesystolicbloodpressureforGroup1is125mmHg,andforGroup2,itis130mmHg.Whichofthefollowinginterpretationsismostsupportedbytheseresults?(A)Thedrugsignificantlyloweredbloodpressurecomparedtotheplacebo.(B)Thedrughadnoeffectonbloodpressure.(C)Theplacebosignificantlyloweredbloodpressure.(D)Theinitialdifferenceinbloodpressurebetweenthegroupswastoolargetobemeaningful.23.Whichofthefollowingprocessesisessentialforthesurvivalofmostplants?(A)Nitrogenfixation(B)Photosynthesis(C)Fermentation(D)Respiration24.Ametalblockisheatedtoahightemperatureandthenplacedinacoolwaterbath.Whichofthefollowingoccursprimarilyduringthetransferofheatbetweentheblockandthewater?(A)Electronsaretransferredfromthemetalblocktothewatermolecules.(B)Energyistransferredaselectromagneticwavesbetweentheblockandthewater.(C)Energyistransferredthroughthecollisionandmovementofparticles.(D)Thechemicalcompositionofboththeblockandthewaterchanges.25.ThePeriodicTableofElementsisorganizedprimarilybasedon:(A)Thedensityoftheelements.(B)Theatomicmassoftheelements.(C)Thenumberofprotonsinthenucleusoftheatoms.(D)Thecoloroftheelementsintheirpureform.26.Anecologistisstudyingthepopulationdynamicsoftwopredatorandpreyspeciesinanecosystem:wolves(predator)andrabbits(prey).Whichofthefollowingobservationswouldmoststronglysuggestthatthewolfpopulationishavingasignificantimpactontherabbitpopulation?(A)Therabbitpopulationisincreasingatasteadyrate.(B)Therabbitpopulationisdecreasing,whilethewolfpopulationisstable.(C)Therabbitpopulationisincreasing,whilethewolfpopulationisdecreasing.(D)Thepopulationsofbothrabbitsandwolvesareincreasinganddecreasinginacyclicalpattern.27.WhichofthefollowingstatementsisacharacteristicofDNA?(A)Itisprimarilyfoundinthecytoplasmofprokaryoticcells.(B)Itiscomposedofaminoacidslinkedtogether.(C)Itcarriesgeneticinformationusedtodirectproteinsynthesis.(D)Itisasimplesugarthatprovidesenergyforcells.28.Achemistisperforminganexperimenttodeterminethedensityofametalcylinder.Shemeasuresthemassofthecylindertobe200grams.Whenthecylinderisfullysubmergedinwater,thevolumeofwaterdisplacedis25cubiccentimeters.Whatisthedensityofthemetal?(A)0.125g/cm³(B)4.0g/cm³(C)5.0g/cm³(D)8.0g/cm³29.Aresearcherhypothesizesthatincreasedlevelsofcarbondioxide(CO₂)intheatmospherewillleadtohigherratesofphotosynthesisinplants.Totestthishypothesis,shesetsupanexperimentwithtwogroupsofidenticalplants.GroupAisgrowninanenvironmentwithnormalCO₂levels,whileGroupBisgrowninanenvironmentwithelevatedCO₂levels.Allotherfactors(light,water,soil)arekeptidentical.WhichofthefollowingwouldbethebestwaytomeasuretheeffectofCO₂levelsonphotosynthesis?(A)Measuretheheightoftheplantsafteronemonth.(B)Measuretheamountofoxygenproducedbyeachplantoverasetperiod.(C)Measurethemassoftheplantsattheendoftheexperiment.(D)Measurethenumberofleavesoneachplant.30.Whichofthefollowingisanexampleofaconvergentevolutionaryprocess?(A)Theevolutionofwingsinbirdsandbats.(B)Theevolutionofsimilarcolorationinfishlivingindifferentdeep-seaenvironmentsascamouflage.(C)Theevolutionofsimilarlimbstructuresinhumansandwhales.(D)TheevolutionofdifferentbeakshapesinDarwin'sfinchesadaptedtodifferentfoodsources.第四部分31.Anastronomerobservesadistantgalaxyandnoticesthatthelightemittedfromitisshiftedtowardslongerwavelengths(redshift).Whichofthefollowinginterpretationsismostconsistentwiththisobservation?(A)Thegalaxyisemittingprimarilyinfraredradiation.(B)ThegalaxyismovingawayfromtheEarth.(C)ThegalaxyismovingtowardstheEarthathighspeed.(D)Thelightfromthegalaxyhasbeendimmedbyinterstellardust.32.Asampleofpurewaterisheatedfrom0°Cto100°C.Whichofthefollowingstatementsaccuratelydescribesthebehaviorofthewatermoleculesduringthisprocess?(A)Theaveragekineticenergyofthemoleculesdecreases.(B)Themoleculesmovefromaliquidstatetoagaseousstatewithoutchangingtheirkineticenergy.(C)Theaveragekineticenergyofthemoleculesincreases,andsomemoleculesmaybreakfreefromtheliquidphasetobecomegas.(D)Themoleculesstopmovingentirelyat100°C.33.Aresearcherisstudyingthegeneticbasisofaparticulartraitinfruitflies.Shediscoversthatthetraitisdeterminedbyasinglegenewithtwoalleles,onedominant(A)andonerecessive(a).Sheperformsacrossbetweentwohomozygousdominantflies(AAxAA).Whatistheexpectedgenotypeandphenotyperatiooftheiroffspring?(A)100%AA(genotype),100%dominantphenotype(B)50%AA,50%Aa(genotype),100%dominantphenotype(C)25%AA,50%Aa,25%aa(genotype),75%dominantphenotype,25%recessivephenotype(D)100%Aa(genotype),50%dominantphenotype,50%recessivephenotype34.Whichofthefollowingstatementsbestexplainswhywaterisconsideredapolarmolecule?(A)Ithasahighboilingpointduetostrongionicbondsbetweenwatermolecules.(B)Itconsistsoftwohydrogenatomsbondedtooneoxygenatominalineararrangement.(C)Theoxygenatomhasaslightlynegativecharge,andthehydrogenatomshaveslightlypositivecharges,creatingadipolemoment.(D)Itreadilydissolvesmanyioniccompoundsduetoitshighelectronegativity.35.Anengineerisdesigningabridge.Sheneedstocalculatetheforcerequiredtosupporttheweightofacarpassingoverthebridge.Whichofthefollowingprinciplesismostrelevanttothiscalculation?(A)Bernoulli'sprinciple(B)Pascal'sprinciple(C)Newton'ssecondlawofmotion(F=ma)(D)Thelawofconservationofenergy36.Abiologistisstudyingtheprocessofmeiosisinacell.WhichofthefollowingeventsoccursduringmeiosisII,similartomitosis?(A)Synapsisofhomologouschromosomesandcrossingover.(B)Formationoftetradsandalignmentofhomologouspairsatthemetaphaseplate.(C)Separationofsisterchromatidsandformationoffourhaploidcells.(D)Formationofthenuclearenvelopeanddecondensationofchromosomes.37.Aweatherforecastmentionsthattheairpressureisfallingandthetemperatureisrising.Whichofthefollowingweatherconditionsismostlikelytooccur?(A)Clearskies(B)Rain(C)Snow(D)Hail38.Whichofthefollowingisaprimaryfunctionoftheribosomesinacell?(A)ReplicationofDNA(B)TranscriptionofDNAintoRNA(C)TranslationofRNAintoproteins(D)Packagingofproteinsintovesicles39.Achemististitratingasolutionofhydrochloricacid(HCl)withasolutionofsodiumhydroxide(NaOH).Whichofthefollowingindicatorswouldbemostsuitablefordeterminingtheendpointofthetitration,wheretheacidandbasehavecompletelyneutralizedeachother?(A)Methylorange(changescolorinthepHrange3.1-4.4)(B)Phenolphthalein(changescolorinthepHrange8.2-10.0)(C)Bromothymolblue(changescolorinthepHrange6.0-7.6)(D)Litmus(changescolorbelowpH7andabovepH8)40.Anobjectisdroppedfromatallbuilding.Assumingnoairresistance,whichofthefollowingstatementsaccuratelydescribesitsmotionafteritisreleased?(A)Theobjectinitiallymovesupwardandthendownward,acceleratingduetogravitythroughout.(B)Theobjectinitiallymovesdownward,acceleratingduetogravity,withitsaccelerationincreasingovertime.(C)Theobjectmovesataconstantvelocity,astherearenoforcesactingonitafterrelease.(D)Theobjectdoesnotmovebecauseitisatrestuntilithitstheground.---试卷答案1.(A)解析思路：题目直接给出了两组数据的平均值，比较平均值即可得出结论。F2组（50cm）的平均高度高于F1组（45cm），因此可以得出F2比F1更有效。选项A正确。2.(C)解析思路：BbxBb的遗传交叉，根据孟德尔遗传定律，后代基因型组合为：25%BB,50%Bb,25%bb。选项C正确。3.(B)解析思路：半衰期是5年，意味着每5年剩余量减半。100克->50克(5年后)->25克(10年后)->12.5克(15年后)->6.25克(20年后)。20年经过了4个半衰期，剩余100*(1/2)^4=6.25克。最接近的选项是25克，这通常表示计算中的近似或选项错误，但根据严格计算6.25克，没有选项匹配，题目可能存在问题。若必须选择，可能题目意在考察3个半衰期即25克的概念，但严格来说4个半衰期应为6.25克。按最接近选项原则，选B。4.(C)解析思路：温室效应是指大气层吸收并重新辐射地球向外发出的红外辐射，导致地球表面温度升高。选项C准确描述了这一点。5.(B)解析思路：使用匀加速直线运动公式v=u+at，其中v=30m/s,u=0m/s(初始速度),t=6s。代入计算a=(30m/s-0m/s)/6s=5m/s²。选项B正确。6.(D)解析思路：根据波义耳定律（恒温下，PV=常数），P1V1=P2V2。已知P1=1.0atm,V1=2.0L,V2=0.5L。求P2=(P1V1)/V2=(1.0atm*2.0L)/0.5L=4.0atm。选项D正确。7.(C)解析思路：化学变化的本质是生成了具有不同化学性质的新物质。例如，铁生锈生成氧化铁，燃烧产生二氧化碳和水。选项C描述了化学变化的特征。8.(A)解析思路：在垂直上抛运动中，到达最高点时，物体的瞬时速度为零，因为它的运动方向发生了改变，但即将开始向下运动。选项A正确。9.(D)解析思路：蒸腾作用是指植物通过叶片将水分蒸发到大气中。这是水循环的重要环节，属于陆地水循环的一部分。选项D正确。10.(A)解析思路：实验结果（音乐组分数略高）可能受到参与者动机的影响。音乐组的参与者可能因为心情更好或更受激励而表现更好，这并非音乐本身直接作用的结果。选项A指出了这种潜在混淆因素。11.(A)解析思路：根据地层学原理，“下老上新”。LayerA在LayerB之上，因此LayerA必然是较年轻的地层。选项A正确。12.(D)解析思路：[注意：题目描述了电路但未提供图示，此题解答基于标准并联电路分析]。总电流I=3A。R1的电压V1=I*R1=3A*4ohms=12V。R2和R3并联，两端电压相等，即V2=V3=V1=12V。通过R2的电流I2=V2/R2=12V/6ohms=2A。通过R3的电流I3也为2A(因为未知R3值，但总电流为3A，I=I2+I3，所以I3=3A-2A=1A)。题目问的是通过R2的电流，为2A。选项D(1.80A)可能是题目或选项设置错误，最接近的合理答案应为2A。按题目给选项，若必须选一个，D最为接近，但严格计算应为2A。13.(C)解析思路：地球绕太阳公转的轨道是近似圆形的，但地球的自转轴相对于其公转轨道平面有一个约23.5度的倾角。这个倾角导致太阳光在不同季节以不同角度照射地球的同一地点，使得接收到的太阳辐射强度和日照时间发生变化，从而形成季节。选项C正确。14.(D)解析思路：初始人口N₀=100，doublingtime=30minutes。总时间t=4.5hours=4.5*60=270minutes。经历的半衰期次数n=t/doublingtime=270min/30min=9。最终人口N=N₀*2ⁿ=100*2⁹=100*512=51,200bacteria。选项D正确。15.(C)解析思路：线粒体是细胞内的“能量工厂”，其主要功能是通过细胞呼吸过程（如克雷布斯循环和电子传递链）将有机物（如葡萄糖）中储存的化学能转化为细胞可利用的能量形式——ATP。选项C正确。16.(B)解析思路：化学方程式为2H₂+O₂→2H₂O。根据题意，4gramsH₂反应。H₂的摩尔质量为2g/mol，所以4gH₂是4g/2g/mol=2molesH₂。根据方程式，2molesH₂需要1moleO₂并生成2molesH₂O。H₂O的摩尔质量为2+16=18g/mol。因此，2molesH₂O的质量为2*18g/mol=36grams。选项B正确。17.(A)解析思路：典型的光合作用速率与光照强度关系图显示，在光饱和点之前，速率随光照强度增加而近似线性增加。当光照强度非常高时，速率达到一个最大值并趋于平稳，因为其他因素（如CO₂浓度、叶绿素含量）成为限制因素。选项A最符合这一预期。18.(C)解析思路：细胞膜的主要功能是作为细胞的边界，控制物质进出细胞，维持细胞内环境的稳定。它具有选择透过性，允许某些物质通过，阻止其他物质通过。选项C正确。19.(B)解析思路：空气阻力（流体阻力）主要作用于物体相对于流体的速度。对于水平抛射或匀速直线运动，空气阻力主要影响水平方向的速度分量，使其逐渐减小。垂直方向上，空气阻力会减小物体下落或上升的加速度（小于g），但不会改变水平方向的速度分量（在无初速度分量情况下）。选项B正确。20.(A)解析思路：动量p=mv。两个物体质量相同(m相同)，ObjectA的速度(v)大于ObjectB(v=0)。因此，ObjectA的动量(p=m*10m/s)大于ObjectB的动量(p=m*0m/s=0)。选项A正确。21.(A)解析思路：根据牛顿万有引力定律和天体运动理论，维持行星围绕恒星做近似圆周运动的是恒星对行星的引力，这个引力提供了行星所需的向心力。选项A正确。22.(A)解析思路：比较两组干预后的结果。Group1(药物)的平均血压从130降至125(下降5mmHg)。Group2(安慰剂)的平均血压从128保持在130(上升2mmHg)。Group1血压下降幅度更大，且变化方向符合药物预期。虽然初始血压略有差异，但药物组干预后的血压显著低于安慰剂组。选项A最合理。23.(B)解析思路：光合作用是绿色植物、藻类和某些细菌利用光能将二氧化碳和水转化为有机物（如葡萄糖）并释放氧气的过程。这是植物生长和获取能量的基础，对大多数植物生存至关重要。选项B正确。24.(C)解析思路：热量传递的三种主要方式是传导、对流和辐射。当金属块放入水中时，热量通过两种方式主要传递：金属块与水分子之间通过碰撞进行能量交换（传导），以及水分子自身由于温度差异而发生流动（对流）。选项C描述了传导和对流的机制。严格来说，辐射也可能发生，但不是主要方式。在选择题中，C是最符合主要机制的描述。25.(C)解析思路：元素周期表是按照元素的原子序数（即原子核中质子的数量）递增的顺序排列的。这是元素周期律的基础，决定了元素的化学性质周期性变化。选项C正确。26.(B)解析思路：捕食者-猎物关系通常呈现周期性波动。如果狼（捕食者）数量增加并对兔子（猎物）造成压力，兔子的数量通常会下降。题目描述狼数量稳定，而兔子数量下降，