请教一道gwd的语法-13-8

fhhrewr

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

of what

at which

我觉得应该选C，that作price的定语，就是the price that the manufacturer charges for another product.

price of what总感觉有些怪。请大家指点！谢谢！