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. 其他的两个选项很好排除，就不写了，答案是c，用are?不明白 句子说的是1972年的条约，后面也用了reduced..为何又用了are?? a. reduced the amount of phosphates that municipalities had been allowed to dump b. reduced the phosphate amount that municipalities had been dumping c. reduced the amount of phosphates that municipalities are allowed to dump | 
