求解，一道没有破解的PREPSC题,hair-growth

今天做PREP模拟，有道题错了，查了PREP破解，没有。只好发贴求教！多谢

A new hair-growth 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

我选了e，答案c。这是不是一个倒装，that修饰price,后置。

另外，price 后面是否应该用at which? 感觉charge the price for sth,应该不用at.

请更正。谢谢

你说的对.charge the price(e.g. 10 dollars) for sth.短语中不用at.

不是倒装.是定语从句.