【IfthepopulationoftheEarthkeepson__61._(increase)atitspresentrate，therewill__62._(event)notbeenoughresourceslefttosupportlifeontheplanet.Bythemiddleofthe21stcentury，ifpresenttrendscontinue，we__63_(use)upall】

　　IfthepopulationoftheEarthkeepson__61._(increase)atitspresentrate，therewill__62._(event)notbeenoughresourceslefttosupportlifeontheplanet.Bythemiddleofthe21stcentury，ifpresenttrendscontinue，we__63_(use)upalltheoilthatdriveourcarsforexample.EvenifscientistsdevelopnewwaysoffeedingthehumanracethecrowdedconditionsonEarthwillmakeit__64 (necessity)forustolookforsomewhereelse.But__65._oftheotherplanetsinoursolarsystemarecapableofsupportinglifeatpresent.Onepossiblesolutiontotheproblem，__66._，hasrecentlybeensuggestedbyanAmericanscientistProfessorCarlSagan.Sagarnbelievesthat__67._theEarth’sresourcesarecompletely__68_(exhaust)，itwillbepossibletochangetheatmosphereofVenusand__69_createanewworldalmostaslargeasEarthitself.Somethingisdifficult.VenusismuchhotterthantheEarth.__70._，thereisonlyatinyamountofwaterthere.