题目内容：
Thewaterwedrinkanduseisrunningshortintheworld.Weallhavetolearnhowtostopwastingourlimitedwater.Oneofthestepsweshouldtakeistofindwaysofreusingit.Experimentshavealreadybeendoneinthisfield.Todayinmostlargecities,freshwaterisusedonlyonce,thenitrunsintowastesystem.Butitispossibletopipetheusedwatertoapurifyingfactory.Thereitcanbefilteredandtreatedwithchemicalssothatitcanbeusedagain,justasitwerefreshfromaspring.
Butevenifeverylargecitypurifiedandreuseditswater,westillwouldnothaveenough.Thenwecouldturntotheoceans.Allwe’dhavetodotomakeuseoftheseawateronearthistogetridofthesalt.Thisprocessiscalleddesalinization,anditisalreadyinuseinmanypartsoftheworld.
Thewaytostopwastingourlimitedwateristo________.
A.doexperimentswithwater
B.purifytheusedwaterandreuseit
C.usefreshwateronceagain
D.makeuseofseawater
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