16. If we had taken 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
16. B考查错综时间的虚拟语气。，据从句的谓语had taken effective measures可知，从句表示与过去事实相反，因此条件从句用过去完成时。根据时间now可知，主句与现在的事实相反，所以用"would+动词原形"的形式。故选B。
题目来源：2015全品语法快练高中英语新课标 > 训练23 情态动词和虚拟语气
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