Ifthepopulationoftheearthgoesonincreasingatitspresentrate,therewilleventuallynotbeenoughresourceslefttosustainlifeontheplanet.Bythemiddleofthe21stcentury,ifpresenttrendscontinue,wewillhaveusedupalltheoilthatdrivesourcars,forexample.Evenifscientistsdevelopnewwaysoffeedingthehumanrace,thecrowdedconditionsonearthwillmakeitnecessaryforlustolookforopenspacesomewhereelse.Butnoneoftheotherplanetsinoursolarsystemarecapableofsupportinglifeatpresent.Onepossiblesolutiontotheproblem,however,hasrecentlybeensuggestedbyAmericanscientist,ProfessorCarlSagan. Saganbelievesthatbeforetheearth'sresourcesarecompleetelyexhausteditwillbepossibletochangetheatmophereofVenusandsocreateanewworldalmostaslargeasearthitself.ThedifficultisthatVenusismuchhotterthantheearthandthereisonlyatinyamountofwaterthere. Saganproposesthatalgaeorganismsthatcanliveinextremelyhotorcoldatmospheresandatthesametimeproduceoxygen,shouldbebredinconditionsimilartothoseonVenus.Assoonasthishasbeendone,thealgaewillbeplacedinsmallrockets.SpaceshipwillthenflytoVenusandfiretherocketsintotheatmosphere.Inafairlyshorttime,thealgewillbreakdownthecarbondioxideintooxygenandcarbon. Whenthealgaehavedonetheriwork,theatmospherewillbecomecooler,butbeformancansetfootonVenusitwillbeneccessaryfortheoxygentoproducerain.ThesurfaceoftheplanetwillstillbetoohotformantolandonitbuttherainwilleventuallyfallandinafewyearssomethinglikeearthwillbereproducedonVenus. Intelongrun,themostinsolubleproblemcausedbypopulationgrowthonearthwillprobablybethelackof______.
A.a.food B.b.oil C.c.space D.d.resources正确答案C
