题目内容：
Questionsarebasedonthefollowingpassage
Ifthepopulationoftheearthgoesonincreasingatitspresentrate,therewilleventuallynotbeenoughresourceslefttokeepourlivesontheplanet.Bythemiddleofthe21stcentury,ifpresenttrendscontinue,wewillhaveusedupalltheoilthatdrivesourcars.Evenifscientistsdevelopnewwaysoffeedingthehumanrace,thecrowdedconditionsonearthwillmakeitnecessaryforustolookforopenspacesomewhereelse.Butnoneoftheotherplanetsinoursolarsystemarecapableofsupportinglifeatpresent.Onepossiblesolutiontotheproblem,however,hasrecentlybeensuggestedbyAmericanscientist,ProfessorCarlSagan.
Saganbelievesthatbeforetheearth'sresourcesarecompletelyusedupitwillbepossibletochangetheatmosphereofVenusandsocreateanewworldalmostaslargeasearthitself.ThedifficultyisthatVenusismuchhotterthantheearthandthereisonlyatinyamountofwaterthere.Saganproposesthatalgae(藻类)organismsthatcanliveinextremelyhotorcoldatmospheresandatthesametimeproduceoxygen,shouldbebredinconditionsimilartothoseonVenus.Assoonasthishasbeendone,thealgaewillbeplacedinsmallrockets.SpaceshipwillthenflytoVenusandfiretherocketsintotheatmosphere.
Inafairlyshorttime,thealgaewillbreakdownthecarbondioxideintooxygenandcarbon.
Whenthealgaehavedonetheirwork,theatmospherewillbecomecooler,butbeforemanbansetfootonVenusitwillbenecessaryfortheoxygentoproducerain.ThesurfaceoftheplanetwillstillbetoohotformantolandonitbuttherainwilleventuallyfallandinafewyearssomethinglikeearthwillbereproducedonVenus.
Inthelongrun,themostinsolubleproblemcausedbypopulationgrowthonearthwillprobablybethelackof__________. A．food
B．oil
C．space
D．resources
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