HaveyouevernoticedwhenyoursweatdriesitwillleavemarksonyourT-shirt?Themarksaretinybitsofsalt.Ifyougototheseasideyouwillknowthatseawaterdoesthesametoyourclothesbecauseitalsohasalotofsaltinit.

Waterevaporatesfromthesurfaceoftheseabutthesaltremains.Haveyoueverthoughtaboutwhethertheseaswillkeepgettingsaltier?Theanswerisno.Theoceanshavestayedatabout3.5percentsaltcontentforhundredsofmillionsofyears.Itkeepsaconstantlevelofsaltindifferentways.

Pickupaseashellandweighitinyourhand.Itisheavy.Allanimalsneedsodiumtoliveandmostneedcalciumtobuildboneanshells.Theseashelllikeallseaanimalsgetsitssodiumandcalciumfromseawater.Whenanimalsdietheirsaltislockedupinbinsandshellswhichdroptothebottomofthesea.

Seawaterandrocksreactwitheachothernotjustinoneway.Seasaltnotonlyreactswithrocksbutalsoreactswiththerocksoftheoceancrust(壳)andvolcaniclava.Thereactionstakesomeofthesaltinseawaterawayfromthesea.

Theouterhardcrustoftheearthismadeupofaboutadozenhardplatesthatdriftonveryhotsoftrocklikefloatingislandsonasea.Theheatwithintheearthisnotthesameeverywhereandtheplatesmovebecauseoftheheat.Whenanoceanplateknocksintoalandplatethethinnerlandplatefloatsovertheoceanone.TheoceanfloorgetspushedunderanditssaltyrocksshellsandbonesarelostdeepwithinEarth.

Sothat’swhytheseasaresaltybutdon’tgetanysaltier.

57.Whatdoestheunderlinedword“evaporates”inParagraph3mean?

A.turnsintosteamB.turnsintoasolid

C.turnsintofreshwater　D.turnsintoice

58.Fromthepassagewecanknowtheseashells________.

A.getsomemineralsfromseawater

B.takesomesaltfromseawhenseawaterreactswithrocks

C.reactwithrocksoftheoceancrustwhenplatesmove

D.absorbsaltwhentheplatesmove

59.Howmanyreasonsdoesthewritermentionwhytheoceansstayatabout3.5％saltcontent?

A.2B.3C.4D.5

60.What’sthepurposeoftheauthortowritethetext?

A.Warning　B.InformingC.Entertaining　D.Describing