og12-74 即 og11-62 时态的疑问

aannaa

A 1972 agreement between Canada and the United States reduced the amout of phosphates that municipalities had been allowed to dump into the Greak Lakes.

(A) reduced the amout of phosphates that municipalities had been allowed to dump
(B) reduced the phosphate amout  that municipalities had been dumping
(C) reduced the phosphate amout  municipalities have been allowed to dump
(D) reduced the amout of phosphates that municipalities are allowed to dump
(E) reduced the amout of phosphates allowed for dumping by municipalities

这道题选D
我对og的解释有疑惑：
An agreement that occurred in 1972 is correctly described with the past tense verb reduced .Since the dumping continued after the date of the agreement ,the past perfect verb had been allowed should instead be the present are allowed (if the agreement remained in effect when the sentence was written ).

那么，既然the dumping continued,同时这个agreement remained in effect when the sentence was written ， 为什么就一定要用过去式reduced而不用一般现在时来表示agreement的内容？

aannaa

谢谢~~

fdrandom

municipalities are allowed to... 可以理解为客观事实吧。agreement是in 1972，过去时间，有时间标志词的不用想太多