Passage2 Ifthepopulationoftheearthgoesonincreasingatitspresentrate,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. Inthelongrun,themostdifficultproblemcausedbypopulationgrowthonearthwillprobablybethelackof__________.
A.food B.oil C.space D.resources正确答案C
