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

此题不论从句意还是句子结构来说，应该说都不难，但错的同学却很多
引起的争议也不小，不少正确答案的解释也是千奇百怪
说明很多同学的英文基础功底还是不过硬
此题句意简单就不翻译了
为了方便分析，把插入成分per milligram去掉
句子变成
A new hair-growing drug is being sold for three times the price——the drug’s maker charges for another product with the same active ingredient.
在空格中应该填入什么呢
答案是that
that引导定语从句，修饰price，that在从句中做charges的宾语（charge的用法：charges ten dollars for a haircut）
为什么选that？
看以下例题
tom的年龄是jerry的3倍，

这样的句意通常可以有三种表示方法：
1 tom is twice older than jerry
2 tom is 3 times as old as jerry
3 tom is 3 times the age of jerry
可以看出，此题和第3种的表达方法类似，只不过把后置定语of jerry改成定语从句而已
此题最容易误选的of what
为什么错？
what = the thing that 在此题中=the price that
选of what则意味
3 times the price of the price that the drug's maker charges for
错误已经很明显了

此题的争议也可以告一段落了