问题补充：
If we had taken such effective measures much earlier, the river so seriously now.
A. is not polluted
B. would not be polluted
C. had not been polluted
D. would not have been polluted
答案：
A. is not polluted
(答案→)B. would not be polluted
C. had not been polluted
D. would not have been polluted
解析：本句从句与过去的事实相反，而主句与现在的事实相反，这里now是解题关键。句意：如果我们早些采取如此有效的措施，这条河现在就不会被严重地污染了。根据句意选B。
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