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. Thispassagemainlytellsusabout__________. 查看材料
A.the analysis of the salinity of ocean waters
B.the causes of the variation in oceanic salinity
C.the importance of the changes in oceanic salinity
D.the different forms of salts in ocean waters
正确答案是B