2.A 1972 agreement between Canada and the United States reduced the amount of phosphates that municipalities had been allowed to dump into the Great Lakes.
(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
(E) reduces the amount of phosphates allowed for dumping by municipalities

I think the answer can be either A or D.
The answer is given to be A
Please specify reason.
Ratheen

It should be D. You cannot reduce the amount of phosphates that 'had been allowed' because that dumping has already been done. You can only reduce the amount that can be dumped from now on. Only D makes it clear.