We must do something to factories waste water into the river before it is cleaned. A. st

问题补充：

We must do something to factories waste water into the river before it is cleaned.

A. stop; pouring .

B. prevent; to pour

C. keep; pouring

D. stop; to pour

答案：

(答案→)A. stop; pouring .

B. prevent; to pour

C. keep; pouring

D. stop; to pour

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