白勇一道题 关于比较级的用法

In the 1980's the rate of increase of the minority population of the United States was nearly twice as fast as the 1970's

A twcie as fast as

B twice as fast as it was in

C twice what it was in

D two times faster than that of

E two times greater than

我想问为什么选C不选B呢， B不是一个没有省略的方式么，It指代the rate of increase...

解释说这是一个句子不能与前面主句（一个名词）比较，但是 我们很多题都是从句都是

一个句子的，例如 The guiding principles of the tax plan released by the Treasury

Department could have even a greater significance for the economy than do the

particulars of the plan.为什么这里这个do不省略。

什么时候从句的谓语不能省要是一个句子，什么时候从句要是一个名词性从句呢？