Passage 2If the population of the earth goes on increasing at its present rate, there will eventuall

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【单选题】Passage2Ifthepopulationoftheearthgoesonincreasingatitspresentrate,therewilleventuallynotbeenoughresourceslefttosustainlifeontheplanet.Bythemiddleofthe21stcentury,ifpresenttrendscontinue,wewillhaveusedupalltheoilthatdrivesourcars,forexample.Evenifscientistsdevelopnewwaysoffeedingthehumanrace,thecrowdedconditionsonearthwillmakeitnecessaryforustolookforopenspacesomewhereelse.Butnoneoftheotherplanetsinoursolarsystemarecapableofsupportinglifeatpresent.Onepossiblesolutiontotheproblem,however,hasrecentlybeensuggestedbyanAmericanscientist,ProfessorCarlSagan.Saganbelievesthatbeforetheearth'sresourcesarecompletelyexhausteditwillbepossibletochangetheatmosphereofVenusandsocreateanewworldalmostaslargeasearthitself.ThedifficultyisthatVenusismuchhotterthantheearthandthereisonlyatinyamountofwaterthere.Saganproposesthatalgaeorganisms,whichcanliveinextremelyhotorcoldatmospheresandatthesametimeproduceoxygen,shouldbebredinconditionssimilartothoseonVenus.Assoonasthishasbeendone,thealgaewillbeplacedinsmallrockets.SpaceshipswillthenflytoVenusandfiretherocketsintotheatmosphere.Inafairlyshorttime,thealgaewillbreakdownthecarbondioxideintooxygenandcarbon.Whenthealgaehavedonetheirwork,theatmospherewillbecomecooler,butbeforemancansetfootonVenusitwillbenecessaryfortheoxygentoproducerain.ThesurfaceoftheplanetwillstillbetoohotformentolandonitbuttherainwilleventuallyfallandinafewyearssomethinglikeearthwillbereproducedonVenus.MancaneventuallylandonVenusonlywhen__________.查看材料

A、thealgaehavedonetheirwork
B、theatmospherebecomescooler
C、thereisoxygen
D、itrainsthere

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