20.   When rates were raised in 1985, postal service officials predicted they would make further rate increases unnecessary for at least three years.

(A) they would make further rate increases unnecessary

(B) they would mean that further rate increases would not be needed

(C) that it would not be necessary for further rate increases

(D) that the increase would make further rate increases unnecessary(D)

(E) further rate increases will not be  needed

请问C为什么不行，it指代后面的“for further rate increases”行不行？

我说一下我的理解

这里it无指代对象

如果it是作为形式主语的话，那么需要用it+不定式来表示

正确答案是D

cause sb/sth to do sth

cause sth for sb

cause sb sth

碰到吃不准的用法可以查LONGMAN，GMAT语法是基于LONGMAN字典的。

谢谢，总结了一下

It作引导词时，有形式主语用法，它有以下相关句型：
一、为形式主语，真正主语由从句充当。
1．  It + be + adj. + that clause
It is clear that the elephant is round and tall like a tree.
很清楚，大象又圆又高，像棵树。

2．  It + be + -ed + that clause
It’s decided that the meeting will start at 6 in the morning.
定于明天六点开会。

3．  It + be + n. + that clause
It’s a pity that he has made such a mistake.
他犯了这样的错误，真遗憾。
 

4．  It + 其它动词 (+宾语) (+状语) + that clause
It worried her a lot that her hair was turning gray. 
   她的头发渐渐变白，这使她非常担心。

6．  It doesn’t matter + clause
It doesn’t matter to me what you do or how you do it.
你做什么或怎么做，对我都没有关系。
 
二、It为形式主语，其它名词性成分做真正的主语。
7．  不定式（或不定式复合结构）做真正主语。
It feels good to lie in the sun or swim in the cool sea.
躺在太阳底下或在海水中游泳，感觉真好。

8．  动名词做真正的主语。
It is no good/ no use/ useless telling him that.
告诉他那事，一点作用也不起。

9．  It takes/ took sb. some time to do sth.
It takes us 30 hours to get there by train.
我们乘火车走了30小时才到那儿。