The air in big cities ______ very dirty by factories.
The air in big cities ______ very dirty by factories.
A.are often made B.is often made C.have often made D.has often made我想选b,可答案为什么给c啊,我觉得d都比c好
A.are often made B.is often made C.have often made D.has often made我想选b,可答案为什么给c啊,我觉得d都比c好
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