请问一个题

In the 1980's the rate of increase of the minority population of the U.S was nearly twice as fast as the 1970's

答案是：twice what it was in 

我不知道为什么这个不对：twice as fast as it was in

他的解释是：it was in the 1970's 是个句子，与名字短语the rate of increase 形式不对称。那正确选项不是也有it was in  吗？

我觉得原句有强调FAST的意思，it 指待the rate of increase 。

请教一下，为什么我分析的不对。

1 twice 是表达数量的，如twice as many/much as,  所以 twice as fast as 这儿已经错了。单纯it was in ，看不出问题。

2 twice还可以加名词或what从句，所以答案是 twice what it was in 

个人认为是这样：

不能用fast，因为rate用高低表示，但是有twice as fast as的说法，不要被误导。

In the 1980's 要与In the 1970's对称，且  what it was 表名词=the rate of increase of the minority population of the U.S，从这个意义上讲也达成了对称。

明白了吗？