a new hair-growth drug is being sold for three times the price, per milligram, as the drug's maker charges for another product with the same active ingredient.
A. as
B. than
C. that
D. of what
E. at which
OA: C
I know I should be specific with my question, but in this case I really don't have a clue where to begin. 'at which' is clearly wrong, and using 'as' could equally mean 'while'. I chose 'of what', but it is probably too informal for the gmat. The answer sounds really odd to me. I would appreciate any explanation. Thanks.