Pool experts have determinded the proper levels of chemicals that must-------in pool water to protect the swimmers from infection and the equipment-------corrosion and damage.Chlorine keills harmful organisms,but it won't work properly if the alkalin

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Pool experts have determinded the proper levels of chemicals that must-------in pool water to protect the swimmers from infection and the equipment-------corrosion and damage.Chlorine keills harmful organisms,but it won't work properly if the alkalinty of the water is too----.If the calcium content of the water is inadequate,the water will leach calcium from the walls of the pool,------them
1.A.maintain B.be maintained C.have maintained D.be maintaning
2.A.by B,from C.for D.of
3.A.low B.below C.under D.beneath
4.A.have weakened B.is weakening C.weakening D.weakens

1.选B.这里是一般的情况.maitain维持,保持.游泳池中的学药品的含量必须保持在一个恰当的水平.在这里要用被动语态,是人类保持其含量.
2.选B.and the equipment-------corrosion and damage与前一短语to protect the swimmers from infection 结构是一样的,只不过是省略了一些相同的词而已.
3.选A.如果水中碱的浓度太低,它就不起作用了.
4.选C.表示伴随动作.

BBAC

BBAC

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