GMAT Sentence Correction (SC) | Re: The number of people flying first class on domestic flights rose sharp

The number of people flying first class on domestic flights rose sharply in 1990, doubling the increase of the previous year.

A. doubling the increase of
B. doubling that of the increase in
C. double as much as the increase of
D. twice as many as the increase in
E. twice as many as the increase of


I think it's D.

But OA is A...WHY??

the number of people is countable so many can be used.