--Why does the river smell terrible?--Because large quantities of water.A. have polluted B

问题补充：

--Why does the river smell terrible?

--Because large quantities of water.

A. have polluted

B. is being polluted

C. has been polluted

D. have been polluted

答案：

A. have polluted

B. is being polluted

C. has been polluted

D. have been polluted

解析：考察主谓一致。large quantities of 放在句首，谓语动词永远都使用复数形式，根据句意可知是被动一意思，故D正确。