请教一题难题！

 A new hair-growing drug is being sold for three time the price , per milligram, as the drug's maker chargs for another product with the same active ingredient.

A as 

B than

C that

D of what

E at which

这题奇怪了，答案是D，可是不是定语从句吗

老外也在争论这道题...

http://www.urch.com/forums/gmat-sentence-correction/40186-set-sc-2.html

http://www.google.com/notebook/public/06457235631081294558/BDRwQSwoQ-tWL19Ah

Here, I vote C.