一道GWD语法，两个版本的答案，到底哪个对

shelleyzheng

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

我看到了两个版本的答案，C和D，到底是哪个呢，请指教

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C that 修饰price，将permilligram视为插入语

原理A is twice B

D的of无法解释