gwd13-8答案是C还是D呢

renprince

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

这题答案是什么呢? 我这里有两个不同的答案.

不过我认为C正确