求一道时态问题，og第77语法题

cuishiliu

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

正确答案是d
我一直很纳闷为什么a不可以
这段话的全文翻译到底是怎么样的呢

had been allowed 我可以理解为是发生在reduced之前
这个条约减少了硝酸盐之前被允许倒入湖里的量

能来个牛牛解答下吗？？

旺财财

最简单的理解方式
我们能发布一个法律去改变以前的历史事件（had been allowed ）么、？
它们既然是had been allowed了，已经发生在这个法律颁布之前，属于历史，怎么被改变、。？
我们要改变的只能是这个法律为坐标向后的事实对不对~~~
这是个切入点~~
hope that helps

cuishiliu

thx dear。。。。。。