Poolexpertshavedetermindedtheproperlevelsofchemicalsthatmust-------inpoolwatertoprotecttheswimmersfrominfectionandtheequipment-------corrosionanddamage.Chlorinekeillsharmfulorganisms,butitwon'tworkproperlyifthealkalintyofthewateristoo----.Ifthecalciumcontentofthewaterisinadequate,thewaterwillleachcalciumfromthewallsofthepool,------them
1.A.maintainB.bemaintainedC.havemaintainedD.bemaintaning
2.A.byB,fromC.forD.of
3.A.lowB.belowC.underD.beneath
4.A.haveweakenedB.isweakeningC.weakeningD.weakens