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1、每月不定期更新，增加新的阅读讲解，敬请关注http://www.docin.com/mydoc-737508-1.html&folderId=48258GMAT题目讲解，2010-4-3In1994,ateamofscientistsledbyDavidMckaybeganstudyingthemeteoriteALH84001,whichhadbeenLinediscoveredinAntarcticain1984.(5)Twoyearslater,theMcKayteamannouncedthatALH84001,whichscientistsgenerallyagreeoriginat
2、edonMars,containedcompellingevidencethatlifeonceexistedonMars.(10)ThisevidenceincludesthediscoveryoforganicmoleculesinALH84001,thefirsteverfoundinMartianrock.Organicmolecules—complex,carbon-basedcompounds—formthebasisfor(15)terrestriallife.TheorganicmoleculesfoundinALH84001arepolycyclicaromatichydr
3、ocarbons,orPAH’s.Whenmicrobesdie,theirorganicmaterialoftendecaysintoPAH’s.(20)SkepticismabouttheMcKayteam’sclaimremains,however.Forexample,ALH84001hasbeenonearthfor13,000years,suggestingtosomescientiststhatitsPAH’smighthave(25)resultedfromterrestrialcontamination.However,McKay’steamhasdemonstratedt
4、hattheconcentrationofPAH’sincreasesasonelooksdeeperintoALH84001,contrarytowhatonewould(30)expectfromterrestrialcontamination.Theskeptic’sstrongestargument,however,isthatprocessesunrelatedtoorganiclifecaneasilyproducealltheevidencefoundbyMcKay’steam,(35)includingPAH’s.Forexample,starformationproduce
5、sPAH’s.Moreover,PAH’sfrequentlyappearinothermeteorites,andnooneattributestheirpresencetolifeprocesses.(40)YetMcKay’steamnotesthattheparticularcombinationofPAH’sinALH84001ismoresimilartothecombinationsproducedbydecayingorganismsthantothoseoriginatingformnobiologicalprocesses.5Thepassageassertswhicho
6、fthefollowingabouttheclaimthatALH84001originatedonMars?ItwasinitiallyproposedbytheMcKayteamofscientists.Itisnotamatterofwidespreadscientificdispute.IthasbeenquestionedbysomeskepticsoftheMcKayteam’swork.IthasbeenunderminedbyrecentworkonPAH’s.ItisincompatiblewiththefacethatALH84001hasbeenonEarthfor13
7、,000years.6TheprimarypurposeofthepassageistodescribenewwaysofstudyingthepossibilitythatlifeonceexistedonMarsrevise改进atheoryregardingtheexistenceoflifeonMarsinlightofnewevidencereconcile调和了conflictingviewpoi
