刘震民改错精解P60 E4-27
A 1972 agreement between Canada and the US reduced the amount of phosphates that municipalities had been allowed to dump into the Great Lake.
这里为什么要用reduce的过去时呢？ 虽然是1972年订的法规，但是“法规”本身的内容，不可以用一般现在时么？ 如果是principle, law， 又该怎样呢？
The principle found in 1990 states/ stated ........?
谢谢指点~ | 
