题目内容：
根据以下材料，回答题Whenweanalyzethesaltsalinity(盐浓度)ofoceanwaters，wefindthatitvariesonlyslightlyfromplacetoplace．Nevertheless，someofthesesmallchangesareimportant．Therearethreebasicprocessesthatcauseachangeinoceanicsalinity．Oneoftheseisthesubtractionofwaterfromtheoceajlbvmeansofevaporation．Inthismanner，thesalinityisincreased，sincethesaltstaysbe—hind．Ifthisiscarriedtotheextreme，ofcourse，whitesaltwouldbeleftbehind；this，bytheway．ishowmuchofthetablesaltweuseisactuallyobtained．
Theoppositeofevaporationisprecipitation，suchasrain，bywhichwaterisaddedtotheo—cean．Heretheoceanisbeingdilutedsothatthesalinityisdecreased．Thismayoccurinareasofhighrainfallorincoastalregionswhereriversflowintotheocean．Thussalinitymaybeincreasedbythesubtractionofwaterbyevaporation，ordecreasedbytheadditionoffreshwaterbyprecipitation．
Normallythanitisininhotregionswherethesunisverystron9，theoceansalinityissomewhathigherpartsoftheworldwherethereisnotasmuchevaporation．Similarly，incoastalregionswhereriversdilutethesea．salinityissomewhatlowerthaninotheroceanicareasAthirdprocessbywhichsalinitymaybealteredisassociatedwiththeformationandmeltingofseaice．Whenseawaterisfrozen，thedissolvedmaterialsareleftbehind．Inthismanner，seawaterdirectlybeneathfreshlyformedseaicehasahighersalinitythanitdidbeforetheiceappeared．Ofcourse，whenthisicemelts，itwilltendtodecreasethesalinityofthesurroundingwater．IntheWeddellSea，thedensestwaterintheoceanisformedasaresultofthis
freezingpro—cess,whichincreasesthesalinityofcoldwater.Thisheavywatersinksandisfoundinthedeeperportionoftheoceansoftheworld.______
Thispassagemainlytellsusabout A．theanalysisofthesalinityofoceanwaters
B．thecausesofthevariationinoceanicsalinity
C．theimportanceofthechangesinoceanicsalinity
D．thedifferentformsofsaltsinoceanwaters
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答案解析：