题目内容：
根据以下材料，回答题Ifthepopulationoftheearthgoesonincreasingatitspresentrate,therewilleventuallynotbe.enoughresourceslefttosustainlifeontheplanet.Bythemiddleofthe21stcentury,ifpresenttrends
continue,wewillhaveusedupalltheoilthatdrivesourcars,forexample.Evenifscien-tistsdevelopnewwaysoffeedingthehumanrace,thecrowdedconditionsonearthwillmakeitnecessaryforustolookforopenspacesomewhereelse.Butnoneoftheotherplanetsinoursolar
systemarecapableofsupportinglifeatpresent.Onepossiblesolutiontotheproblem,however,hasrecentlybeensuggestedbyanAmericanscientist,ProfessorCarlSagan.
Saganbelievesthatbeforetheearth'sresourcesarecompletelyexhausteditwillbepossibletochangetheatmosphereofVenusandsocreateanewworldalmostaslargeasearthitself.The
difficultyisthatVenusismuchhotterthantheearthandthereisonlyatinyamountofwaterthere.Saganproposesthatalgaeorganisms,whichcanliveinextremelyhotorcoldatmospheresand
atthesametimeproduceoxygen,shouldbebredinconditionssimilartothoseonVenus.Assoonasthishasbeendone,thealgaewillbeplacedinsmallrockets.SpaceshipswillthenflytoVenus
andfiretherocketsintotheatmosphere.Inafairlyshorttime,thealgaewillbreakdownthecar-bondioxideintooxygenandcarbon.
Whenthealgaehavedonetheirwork,theatmospherewillbecomecooler,butbeforemancansetfootonVenusitwillbenecessaryfortheoxygentoproducerain.Thesurfaceoftheplanetwillstill
betoohotformentolandonitbuttherainwilleventuallyfallandinafewyearssome-thinglikeearthwillbereproducedonVenus.
Inthelongrun,themost'difficultproblemcausedbypopulationgrohonearthwillprobablybethelackof A．food
B．oil
C．space
D．resources
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