我觉得吧，这里不用过去完成时的原因就2个：
1. “the amount of phosphates that municipalities are allowed to dump into the Great Lakes” 这个就是1972年法令的本身内容，所以就不要考虑其他时态了；
2. 即使你想考虑过去完成时，那么必须要有过去完成时存在的要素：一个明显的时间对比，来强调一个动作发生在另一个动作之前。这里1972年是个绝对的时间概念，并没有做时间上的对比，我这里给你个另一题的例子做参考，我们来看看怎么样的时间对比概念算是形成了两个动作在时间上的对比：
According to some analysts, the gains in the stock market reflect growing confidence
that the economy will avoid the recession that many had feared earlier in the year and instead come in for a “soft landing,” followed by a gradual increase in business activity.
(A) that the economy will avoid the recession that many had feared earlier in the year and instead come
(B) in the economy to avoid the recession, what many feared earlier in the year, rather to come
(C) in the economy’s ability to avoid the recession, something earlier in the year many had feared, and instead to come
(D) in the economy to avoid the recession many were fearing earlier in the year, and rather to come
(E) that the economy will avoid the recession that was feared earlier this year by many, with it instead coming
该题目答案是A，earlier in the year明显的形成了两个动作在时间上的对比，强调了一个动作发生在另一个动作之前。如果楼主有心，可以把OG用到过去完成时的例子贴出来，我们来看看是不是都有这么个时间对比的元素在。
还是我的一些愚见，望大牛们补充指正，呵呵~
-- by 会员 popup (2012/2/4 11:37:00)
thx~