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.Youshoulddecideonthebestchoiceandthenmarkthecorrespondinglettero

2、ntheAnswerSheet.Questions16to20arebasedonthefollowingpassage:Eighttimeswithinthepastmillionyears,somethingintheEarth'sclimaticequationhaschanged,allowingsnowinthemountainsandthenorthernlatitudestoaccumulatefromoneseasontothenextinsteadofmeltingaway.Eachtime,theenormousicesheetsresultingfromthisc

3、ontinualbuilduplastedtensofthousandsofyearsuntiltheendofeachparticularglacialcyclebroughtawarmerclimate.Scientistsspeculatedthattheseglacialcycleswereultimatelydrivenbyastronomicalfactor:slow,cyclicchangesintheeccentricityoftheEarth'sorbitandinthetiltandorientationofitsspinaxis.Butupuntilaround3

4、0yearsago,thelackofanindependentrecordofice-agetimingmadethehypothesisuntreatable.Thenintheearly1950'sEmirianproducedthefirstcompleterecordofthewaxwingsandawningsoffirstglaciations.Itcamefromaseeminglyoddplace,theseafloor.Single-cellmarineorganismscalled"foraminifera''housethemselvesinshellsmade

5、fromcalciumcarbonate.Whentheforaminiferadie,sinktothebottom,andbecomepartofseafloorsediments,thecarbonateoftheirshellspreservescertaincharacteristicsoftheseawatertheyinhabited.Inparticular,theratioofaheavyisotopeofoxygen(oxygen-18)toordinaryoxygen(oxygen-16)inthecarbonatepreservestheratioofthetw

6、ooxygen’sinwatermolecules.Itisnowunderstoodthattheratioofoxygenisotopesreflectstheproportionoftheworld'swaterlockedupinglaciersandicesheets.Akindofmeteorologicaldistillationaccountsforthelink.Watermoleculescontainingtheheavierisotopetendtocondenseandfallasprecipitationslightlysoonerthanmolecules

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