[Question deleted because it is from a banned source.]

Why is D incorrect? Why is B incorrect? Why is C correct? Can you please explain? I don't see anything wrong with 'a rate of increase being as fast as...'. Thanks.

Choices B and D have the same problem: redundancy.
The rate of increase was greater than it was before: not redundant.
The increase was faster than it was before: not redundant.
The rate of increase was faster than it was before: redundant!

Learn to look for this - it's harder to spot, because you wouldn't think it wrong in SPOKEN language.

For the same reason: A car can be fast, or it can move with a high speed. But it can't move with "a fast speed", at least not in formal written English.

Ron,

Can we attack this Q like this?

rate of increase should be a number. So it cant be faster? can be greater (A,B and D out)

rate of increase in 1980's must be compared with the rate of increase in 1970's (A & E compare ROI in 80's with the 1970's)

I was torn between B and C and picked B. I over analyzed the question and actually thought that the rate of increase can be faster.

Please remember to read (and follow!) the forum guidelines before posting. 1000SC is a banned source. Ron was responding to questions at 2:30 in the morning and probably just didn't notice, so you got lucky this time. Please don't post from banned sources in future, though!