GMAT Sentence Correction (SC) | OG 13th edition Q 77 SC

Hi all

The question reads: :" A 1972 agreement between Canada and the United statesreduced the amount of phosphates that municipalities had been allowed to dump into the Great Lakes.

I have managed to narrow down the options to two:

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

The correct answer is D, but I am trying to understand what is wrong with A. Ideally, the first sentence
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