考博英语模拟试题与参考答案及解析

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1、考博英语模拟试题PartIListeningComprehension(15points)(略)PartIIReadingComprehension(40%)Directions:Thereare4readingpassagesinthispart.Eachpassageisfollowedbysomequestionsorunfinishedstatements.ForeachorthemtherearefourchoicesmarkedA,B,CandD.Youshoulddecideonthebestchoice

2、andthenmarkthecorrespondingletterontheAnswerSheet.Questions16to20arebasedonthefollowingpassage:Eighttimeswithinthepastmillionyears,somethingintheEarth'sclimaticequationhaschanged,allowingsnowinthemountainsandthenorthernlatitudestoaccumulatefromoneseasontothenext

3、insteadofmeltingaway.Eachtime,theenormousicesheetsresultingfromthiscontinualbuilduplastedtensofthousandsofyearsuntiltheendofeachparticularglacialcyclebroughtawarmerclimate.Scientistsspeculatedthattheseglacialcycleswereultimatelydrivenbyastronomicalfactor:slow,cy

4、clicchangesintheeccentricityoftheEarth'sorbitandinthetiltandorientationofitsspinaxis.Butupuntilaround30yearsago,thelackofanindependentrecordofice-agetimingmadethehypothesisuntreatable.Thenintheearly1950'sEmirianproducedthefirstcompleterecordofthewaxwingsandawnin

5、gsoffirstglaciations.Itcamefromaseeminglyoddplace,theseafloor.Single-cellmarineorganismscalled"foraminifera''housethemselvesinshellsmadefromcalciumcarbonate.Whentheforaminiferadie,sinktothebottom,andbecomepartofseafloorsediments,thecarbonateoftheirshellspreserve

6、scertaincharacteristicsoftheseawatertheyinhabited.Inparticular,theratioofaheavyisotopeofoxygen(oxygen-18)toordinaryoxygen(oxygen-16)inthecarbonatepreservestheratioofthetwooxygen’sinwatermolecules.Itisnowunderstoodthattheratioofoxygenisotopesreflectstheproportion

7、oftheworld'swaterlockedupinglaciersandicesheets.Akindofmeteorologicaldistillationaccountsforthelink.Watermoleculescontainingtheheavierisotopetendtocondenseandfallasprecipitationslightlysoonerthanmoleculescontainingthelighterisotope.Hence,aswatervaporevaporatedfr

8、omwarmoceansmovesawayfromitssource,itsoxygen-18returnsmorequicklytotheoceansthandoesitsoxygen-16.Whatfallsassnowondistanticesheetsandmountainglaciersisrelativelydeple