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时间:2020-03-15
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1、考博英语模拟试题PartIListeningComprehension(15points)(略)PartIIReadingComprehension(40%)Directions:Thereare4readingpassagesinthispart.Eachpassageisfollowedbysomequestionsorunfinishedstatements.ForeachorthemtherearefourchoicesmarkedA,B,CandD.Youshoulddecideonthebestchoicea
2、ndthenmarkthecorrespondingletterontheAnswerSheet.Questions16to20arebasedonthefollowingpassage:Eighttimeswithinthepastmillionyears,somethingintheEarth'sclimaticequationhaschanged,allowingsnowinthemountainsandthenorthernlatitudestoaccumulatefromoneseasontothenextin
3、steadofmeltingaway.Eachtime,theenormousicesheetsresultingfromthiscontinualbuilduplastedtensofthousandsofyearsuntiltheendofeachparticularglacialcyclebroughtawarmerclimate.Scientistsspeculatedthattheseglacialcycleswereultimatelydrivenbyastronomicalfactor:slow,cycli
4、cchangesintheeccentricityoftheEarth'sorbitandinthetiltandorientationofitsspinaxis.Butupuntilaround30yearsago,thelackofanindependentrecordofice-agetimingmadethehypothesisuntreatable.Thenintheearly1950'sEmirianproducedthefirstcompleterecordofthewaxwingsandawningsof
5、firstglaciations.Itcamefromaseeminglyoddplace,theseafloor.Single-cellmarineorganismscalled"foraminifera''housethemselvesinshellsmadefromcalciumcarbonate.Whentheforaminiferadie,sinktothebottom,andbecomepartofseafloorsediments,thecarbonateoftheirshellspreservescert
6、aincharacteristicsoftheseawatertheyinhabited.Inparticular,theratioofaheavyisotopeofoxygen(oxygen-18)toordinaryoxygen(oxygen-16)inthecarbonatepreservestheratioofthetwooxygen’sinwatermolecules.Itisnowunderstoodthattheratioofoxygenisotopesreflectstheproportionofthew
7、orld'swaterlockedupinglaciersandicesheets.Akindofmeteorologicaldistillationaccountsforthelink.Watermoleculescontainingtheheavierisotopetendtocondenseandfallasprecipitationslightlysoonerthanmoleculescontainingthelighterisotope.Hence,aswatervaporevaporatedfromwarmo
8、ceansmovesawayfromitssource,itsoxygen-18returnsmorequicklytotheoceansthandoesitsoxygen-16.Whatfallsassnowondistanticesheetsandmountainglaciersisrelativelydeple
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