Sentence Correction

According to the National Science Foundation, in 2003 there were 198,113 female science and engineering graduate students, almost 42% of the graduate students in those fields, twice as much as 1981.
A twice as much as 1981
B twice as many as 1981
C double the figure for 1981
D double what it was in 1981
E a number double that of 1981’s

This is a question from MGMAT Online CAT, the answer is C[spoiler]

IMO the "for" is inappropriate... any pointers?
[/spoiler]

I think "for" is used as preposition, parallel with in 2003