If we had taken such effective measures much earlier, the river __________so seriously now.A. .is not pollutedB. .would not be pollutedC. .had not been pollutedD. .would not have been polluted

步骤 1：理解题意
题目要求我们选择一个合适的选项来完成句子，句子的前半部分是虚拟语气，表示对过去的假设，后半部分是对现在情况的描述。
步骤 2：分析选项
A. is not polluted - 现在时态，不符合虚拟语气的结构。
B. would not be polluted - 与现在时态的虚拟语气结构相匹配，表示对现在的假设。
C. had not been polluted - 过去完成时，不符合虚拟语气的结构。
D. would not have been polluted - 过去完成时的虚拟语气，表示对过去的假设，不符合题意。
步骤 3：选择正确答案
根据题意，我们需要选择一个与虚拟语气结构相匹配的选项，因此选项B是正确的。