OG 12 SC74 时态问题

jetball

74.    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

OG的解释说从1972就能看出应该用过去时态。这个不是太理解，1972只是说这是那一年的agreement，并不能说一定是那一年reduce的啊？这个agreement可以一直有效，所以应该用一般现在时才对啊。求NN解释！

manxeon

强调那个agreement生效时产生了reduce的作用。

jetball

不是太明白。。。