阅读理解 Passage1 Ifthepopulationoftheearthgoesonincreasingatitspresentrate,therewilleventuallynotbeenoughresourceslefttosustainlifeontheplanet.Bythemiddleofthe21stcentury,ifpresenttrendscontinue,wewillhaveusedupalltheoilthatdrivesourcars,forexample.Evenifscientistsdevelopnewwaysoffeedingthehumanrace,thecrowdedconditionsonearthwillmakeitnecessaryforustolookforopenspacesomewhereelse.Butnoneoftheotherplanetsinoursolarsystemarecapableofsupportinglifeatpresent.Onepossiblesolutiontotheproblem,however,hasrecentlybeensuggestedbyAmericanscientist,ProfessorCarlSagan. Saganbelievesthatbeforetheearth'sresourcesarecompletelyexhausteditwillbepossibletochangetheatmosphereofVenusandsocreateanewworldalmostaslargeasearthitself.ThedifficultisthatVenusismuchhotterthantheearthandthereisonlyatinyamountofwaterthere. Saganproposesthatalgaeorganismsthatcanliveinextremelyhotorcoldatmospheresandatthesametimeproduceoxygen,shouldbebredinconditionsimilartothoseonVenus.Assoonasthishasbeendone,thealgaewillbeplacedinsmallrockets.SpaceshipwillthenflytoVenusandfiretherocketsintotheatmosphere.Inafairlyshorttime,thealgaewillbreakdownthecarbondioxideintooxygenandcarbon. Whenthealgaehavedonetheirwork,theatmospherewillbecomecooler,butbeforemancansetfootonVenusitwillbenecessaryfortheoxygentoproducerain.ThesurfaceoftheplanetwillstillbetoohotformantolandonitbuttherainwilleventuallyfallandinafewyearssomethinglikeearthwillbereproducedonVenus.