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 phosphates amount that municipalities had been dumping
(C) reduces the phosphates amount that 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
答案选D。不明白为什么要用are allowed to dump,如果1972已经成为过去,那么过去的一个agreement为什么能影响现在的排放量呢?