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.
首先过去完成式表示一种比较,常跟一般完成时态连用,原文里并没有与现在完成时态的比较 所以不能用过去完成时态 排除了A B 2.现在完成时态用的时候要有明显的时间标志词 例如since.... within/over the last/past/recent x years 3 这里是指一个agreement 是一个客观事实 所以要用一般现在时态 4E选项allow for dumping by municipalities 这样用意思不对 akward