请教，OG 11th紫皮书62题，时态分析

题目如下：

62.   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

分析如下：

A     Had been allowed should be are allowed

B     The phosphate amount should be the amount of phosphates; the meaning of the sentence is changed by the omission of any form of allow

C     The present tense reduces should be the past tense reduced; the phosphate amount should be the amount of phosphates; have been allowed should be are allowed

D     Correct. The past tense reduced is correctly used in this sentence to describe a past action, and the present tense are allowed is used to describe the present situation.

E      The present tense reduces should be the past tense reduced; allowed for dumping is an incorrect idiom; allowed for dumping by municipalities is awkward

我的问题是：为什么用reduced，而不用reduces？这种主句和从句时态不一致的情况感觉很微妙，不好把握。谢谢

这题引出一种新的时态表达法

就是一句话可以同时出现一般过去时和一般现在时，表示一般现在时的动作发生在一般过去时那个动作的将来，并且受一般过去时那个动作的影响。

虽然这种时态通常可以用过去将来时，即在这里写成would be allowed，但这种写法常常显得ambiguous，因为would也可以用作委婉语，从而不能清晰表达因果关系。

这是一个有争议的og题目。

关于这个问题的时态，我曾经上一个美国人的gmat网站上面看过，大部分人都不认可og的答案。