GMAT Sentence Correction (SC) | Re: A 1972 agreement between Canada and the United States reduce

A 1972 agreement between Canada and the United Statesreduced the amount of phosphates that municipalities had been allowed to dump into the Great Lakes.

There are two actions and they are notdependent and hence past perfect tense need not be used and secondly, the two clauses are joined as a subordinate clauses "using that". Check out the question : galileo-did-not-invent-the-telescope-but-on-hearing-in-111155.html