问BY上的一道比较结构的题

P204的36题

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)twice 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

答案选C,可是不明白B错误的原因，希望高手解释一下，想得头大了