HI abhasjha,

This SC is based on Comparison rules (and by extension, Parallelism rules) and a minor usage/style rule. The tough part will be in avoiding the answers that "sound like how people talk."

1) Comparison: The first part of the sentence offers the first part of the comparison - "the rate of increase (in the 1980s)....was...", so the second part of the comparison must also offer "a rate of increase" or equivalent. Here, the pronoun "it" is enough. Eliminate A, D and E.

2) Usage/Style: In the context of this sentence, a rate can't be described as "fast" when compared to another rate. It CAN be described as a higher or lower rate. Here, the rate in the 80s is essentially higher than the rate in the 70s, so we need to communicate that idea correctly. Eliminate B.

Final Answer: C

GMAT assassins aren't born, they're made,
Rich