[求助]白勇CH7-36

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.

(B) twice as fast as it was in

(C) twice what it was in

B解释：it was in the 1970's是个句子，与名词短语the rate of increase形式不对称。

ANS:C         

请问B的it为什么不能指代 the rate of increase→ twice as fast as the rate of increase was in the 1970's.