请阅读短文，完成此题。 Whenweanalyzethesaltsalinity(盐溶度)ofoceanwaters,wefindthatitvariesonlyslightlyfromplacetoplace.Nevertheless,someofthesesmallchangesareimportant.Therearethreebasicprocessesthatcauseachangeinoceanicsalinity.Oneoftheseisthesubtractionofwaterfromtheoceanbymeansofevaporation.Inthismanner,thesalinityisincreased,sincethesaltstaysbehind.Ifthisiscarriedtotheextreme,ofcourse,whitesaltwouldbeleftbehind;this,bytheway,ishowmuchofthetablesaltweuseisactuallyobtained. Theoppositeofevaporationisprecipitation,suchasrain,bywhichwaterisaddedtotheocean.Heretheoceanisbeingdilutedsothatthesalinityisdecreased.Thismayoccurinareasof highrainfallorincoastalregionswhereriversflowintotheocean.Thussalinitymaybeincreasedbythesubtractionofwaterby-evaporation,ordecreasedbytheadditionoffreshwaterbyprecipitation. Normally,inhotregionswherethesunisverystrong,theoceansalinityissomewhathigherthanitisinotherpartsoftheworldwherethereisnotasmuchevaporation.Similarly,incoastalregionswhereriversdilutethesea,salinityissomewhatlowerthaninotheroceanicareas. Athirdprocessbywhichsalinitymaybealteredisassociatedwiththeformationandmeltingofseaice.Whenseawaterisfrozen,thedissolvedmaterialsareleftbehind.Inthismanner,seawaterdirectlybeneathfreshlyformedseaicehasahighersalinitythanitdidbeforetheiceappeared.Ofcourse,whenthisicemelts,itwilltendtodecreasethesalinityofthesurroundingwater. IntheWeddellSea,thedensestwaterintheoceanisformedasaresultofthisfreezingprocess,whichincreasesthesalinityofcoldwater.Thisheavywatersinksandisfoundinthedeeperportionoftheoceansoftheworld. Itcanbeknownfromthepassagethatincreaseinthesalinityofoceanwateriscausedby__________. 查看材料
A.melting of sea ice
B.precipitation
C.evaporation
D.supplement of salt
正确答案是C