GWD4-Q4

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

请问这题C为什么对，其他选项为什么错。 谢谢大家！！

A. as + n. 意为“作为”,错；

B. times的用法为：times(或数字词，如twice）+ as+ adj.+ as

                  times the n.

                  times what

C.that 引导的定语从句修饰the price

D.应为what

E.at which不如that（我不确定究竟为什么不能选）

charge some money for sth，固定搭配，所以不用at which

D选项中为什么应该是what？用what肯定不是定语从句了，ls可以解释一下吗？