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The air in big cities _____very dirty by factories.

A. are often made         B. is often made       C. have often madeD. has often made

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