IfthepopulationoftheEarthkeepsonincreasingatitspresentratetherewilleventuallynotbeenoughresources(31)________(leave)tosupportlifeontheplanet.Bythemiddleofthe21stcenturyifpresenttrendscontinuewe(32)_________（use）upalltheoilthatdrivesourcarsforexample.Evenifscientistsdevelopnewwaysoffeedingthehumanracethecrowdedconditionsontheearthwillmake(33)______necessaryforustolookforopenspacesomewhereelse.But(34)______oftheotherplanetsinoursolarsystemarecapableofsupportinglifeatpresent.Onepossiblesolution(35)______theproblem(36)_________hasrecentlybeensuggestedbyanAmericanscientistProfessorCarlSagan.Saganbelieves(37)_______beforetheEarth’sresourcesare(38)_________(complete)exhausteditwillbepossibletochangetheatmosphereofVenusandthereforecreateanewworldalmostaslargeastheEarth(39)__________.ThedifficultyisthatVenusismuchhotterthantheEarth.(40)_________thereisonlyatinyamountofwaterthere.