GMAT Sentence Correction (SC) | A new hair-growing drug is being sold for three times

A new hair-growing 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
The answer is c, but what about e? Thank you for your help.
Source: GWD-4-4

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