关于the amount of是否只接不可数名词的问题

勇敢的心2014

13th OG77:
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

答案是D，the amount of后加的是复数名词，
GMAT里面AMOUNT的用法解释是：amount is for undifferentiated masses such assand or water，即amount用于修饰不可分割的物体，即修饰不可数名词。

请问这怎么解释呢？谢谢

勇敢的心2014

自己顶，求解答

xiuyiwei1

你这么一说确实怪怪的，但是这道题并没有用这个知识点来解答呃感觉