The frightening thing is the rate at which it is increasing.这句讲的是污染的增速后半句中为什么要加it?

The frightening thing is the rate at which it is increasing.这句讲的是污染的增速后半句中为什么要加it?
The frightening thing is the rate at which it is increasing.
这句讲的是污染的增速
后半句中为什么要加it?

The frightening thing is the rate at which it is increasing.这句讲的是污染的增速后半句中为什么要加it?
好,这个句子我们来划分成分：简化的说,应该是The frightening thing is the rate.到这里句子就是完整的.后面的成分就是对 the rate的定语从句.那么现在主句我们没有问题了,单看从句.
从句which指的就是rate.那么就是说：It is increasing at the rate.显然,如果,没有It的话,从句就缺少成分.而这里的It就是指污染的速度.翻译过来就是污染的速度一直增加.
你的这个句子应该是一段话挑出来的吧!所以要结合上下文理解.

The frightening thing is the rate at which it is increasing.
这句是which引导的宾语从句，如果不用it的话，那么宾语从句就缺少主语了。 it is increasing.是从句it做主语