Whenever we have a comparison, our first move should be to make sure that it makes sense to compare the two things we are comparing.

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.

Here, we are comparing a rate to a year, which doesn't make a lot of sense: we want to compare the rate in the 1980's to the rate in the 1970's. This is an illogical comparison. We can eliminate A and E right off the bat.

Picking C over B and D largely comes down to concision. All of the answers say the same thing, but one of them says it in less words. On the GMAT, we always want to choose shortest, least clunky answer that 1) makes sense and 2) doesn't change the meaning. So we pick our shortest and least clunky answer, C!