中国問題ちょっと疑問＃３

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

c)が正解ですが、d)の駄目は理由をどなたか教えていただけますか？
どうもrate of increaseの使い方で混乱してます。

http://tools.geocities.co.jp/CollegeLife-Cafe/6302/@geoboard/