题目内容：
根据以下材料，回答题Waterproblemsinthefuturewillbecomemoreintenseandmorecomplex.Ourincreasingpopulationwilltremendouslyincreaseurbanwastes,primarilysewage.Ontheotherhand,increas-ingdemandsforwaterwilldecreasesubstantiallytheamountofwateravailablefordilutingwastes.
Rapidlyexpandingindustrieswhichinvolvemoreandmorecomplexchemicalprocesseswillpro-ducelargevolumesofliquidwastes,andmanyofthesewillcontainchemicalswhicharepoi-sonous.Tofeedourrapidlyexpandingpopulation,agriculturewillhavetobeintensified.Thiswillinvolveever-increasingquantitiesofagriculturechemicals.Fromthis,itisapparentthatdrasticstepsmustbetakenimmediatelytodevelopcorrectivemeasuresforthepollutionproblem.
Therearetwowaysbywhichthispollutionproblemcanbelessened.Thefirstrelatestothetreatmentofwastestodecreasetheirpollutionhazard.Thisinvolvestheprocessingofsolidwastesprior
todisposalandthetreatmentofliquidwastes,oreffluents,topermitthereuseofthewaterorbestreducepollutionuponfinaldisposal.
Asecondapproachistodevelopaneconomicuseforallorapartofthewastes.Farmmanureisspreadinfieldsasanutrientororganicsupplement.Effluentsfromsewagedisposalplantsareused
insomeareasbothforirrigationandforthenutrientscontained.Effluentsfromotherprocess-ingplantsmayalsobeusedasasupplementalsourceofwater.Manyindustries,suchasmeatandpoultryprocessingplants,arecurrentlyconvertingformerwasteproductionintomarketable
byproducts.Otherindustrieshavepotentialeconomicusesfortheirwasteproducts.
Thepurposeofthispassageis A．towarnthereaderofthedwindlingwatersupply
B．toexplainindustrialusesofwater
C．toacquaintthereaderwithwaterpollutionproblems
D．todemonstratevariousmeasurestosolvethepollutionproblem
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答案解析：