tgonza wrote:Why is the answer C instead of B?

Statement: 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) Listed above
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

This is idiomatic.
Have a look at this sentence.
Rate of increase of A is twice of what it is of B.
Rate of increase of A is twice as fast as it is of B.
If you notice, rate of increase twice does not need the word "fast" because it already signifies the meaning without the word "fast".
Or have a look at this.
Rate of increase of A is twice as slow as it is of B. This sentence may not be making sense to you because we can just write that rate of increase of A is half as it is of B. So you can see that there is no need of the word "fast" or "slow" here.

This has been discussed many times recently.

In my opinion,

If the rate is twice - > it is obviously fast. Hence presence of both twice and fast is redundant.