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 [C]

5. at which

看了讨论稿，还是不太明白？我比较倾向预选D能不能说说正确答案C的理由阿？thanks you guys