考博英语模拟试题与答案详解

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

2、letterontheAnswerSheet.Questions16to20arebasedonthefollowingpassage:Eighttimeswithinthepastmillionyears,somethingintheEarth'sclimaticequationhaschanged,allowingsnowinthemountainsandthenorthernlatitudestoaccumulatefromoneseasontothenextinsteadofmeltingaway.Eachtime,theenormousicesheetsresu

3、ltingfromthiscontinualbuilduplastedtensofthousandsofyearsuntiltheendofeachparticularglacialcyclebroughtawarmerclimate.Scientistsspeculatedthattheseglacialcycleswereultimatelydrivenbyastronomicalfactor:slow,cyclicchangesintheeccentricityoftheEarth'sorbitandinthetiltandorientationofitsspina

4、xis.Butupuntilaround30yearsago,thelackofanindependentrecordofice-agetimingmadethehypothesisuntreatable.Thenintheearly1950'sEmirianproducedthefirstcompleterecordofthewaxwingsandawningsoffirstglaciations.Itcamefromaseeminglyoddplace,theseafloor.Single-cellmarineorganismscalled"foraminifera'

5、'housethemselvesinshellsmadefromcalciumcarbonate.Whentheforaminiferadie,sinktothebottom,andbecomepartofseafloorsediments,thecarbonateoftheirshellspreservescertaincharacteristicsoftheseawatertheyinhabited.Inparticular,theratioofaheavyisotopeofoxygen(oxygen-18)toordinaryoxygen(oxygen-16)int

6、hecarbonatepreservestheratioofthetwooxygen’sinwatermolecules.Itisnowunderstoodthattheratioofoxygenisotopesreflectstheproportionoftheworld'swaterlockedupinglaciersandicesheets.Akindofmeteorologicaldistillationaccountsforthelink.Watermoleculescontainingtheheavierisotopetendtocondenseandfall

7、asprecipitationslightlysoonerthanmoleculescontainingthelighterisotope.Hence,aswatervaporevaporatedfromwarmoceansmovesawayfromitssource,itsoxygen-18returnsmorequicklytotheoceansthandoesitsoxygen-16.Whatfallsassnowondistanticesheetsandmountainglaciersisrelativelydeple