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(A) reduced the amount of phosphates that municipalities had been allowed to dump>>

(B) reduced the phosphate amount that municipalities had been dumping

(C) reduces the phosphate amount municipalities have been allowed to dump

(D) reduced the amount of phosphates that municipalities are allowed to dump（A）

(E) reduces the amount of phosphates allowed for dumping by municipalities

I personally believe that the answer  is C. The 1972 agreement only can reduce the amount of phosphate be allowed to dump after 1972 not before 1972. The right tense should be present perfect.

Welcome to discuss

Thanks

I noticed that 1972 is past time but agreement is only aplicable to the future action, isn't it?

Thanks for your explanation anyway.

我想把reduce看作是agreement当时产生的结果更好些。原来municipalities被允许倒100吨，1972的agreement要求最多是50吨，这样就reduced the amount。这不是个渐进过程，而是个瞬间的结果。