－Why does the river smell terrible?－Because large quantities of waterA. have been pollut

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(答案→)A. have been polluted
B. is being polluted
C. has been polluted
D. have polluted
解析：考查时态。根据语境可知，大量的水已经被污染，故用现在完成时态的被动语态。句意：——为什么这条河这么难闻？——因为大量的水已经被污染。故A正确。