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.

1. as
2. than
3. that
4. of what
5. at which             数词+times后接什么这个问题我一直就没有搞清楚，我就选了D，答案是C，请大家指教           

选C的原因是  that the drug’s maker charges for ... 是定语从句修饰price. per

milligram 为插入语。

three times the price, per milligram, that the drug’s maker charges for