求教！【已解决】

33.   A substance derived from the Madagascar periwinkle, which has proved useful in decreasing mortality among young leukemia patients, is cultivated in China as part of a program to integrate traditional herbal medicine into a contemporary system of health care.

(A) A substance derived from the Madagascar periwinkle, which has proved useful in decreasing mortality among young leukemia patients,
   

(B) A derivative, which has proved useful in decreasing mortality among young leukemia patients, of the Madagascar periwinkle,

(C) A Madagascar periwinkle derivative, which has proved useful in decreasing mortality among young leukemia patients,

(D) The Madagascar periwinkle has a derivative which has proved useful in decreasing mortality among young leukemia patients, that

(E) The Madagascar periwinkle, a derivative of which has proved useful in decreasing mortality among young leukemia patients,
  

答案是E，为什么不能选C呢？这是从携隐的语法大全987里找的第33题，请大家给我讲解一下吧，谢谢！