1. 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） (E) The Madagascar periwinkle, a derivative of which has proved useful in decreasing mortality among young leukemia patients,
为什么此题选e,而不是c? | 
