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, 选项E的of which修饰的是什么。我觉得 a derivative是 The Madagascar periwinkle的同位语。但是这样的话 of which是指代什么呢？难道是说 The Madagascar periwinkle 是 The Madagascar periwinkle自己的 derivative？ 不弄明白睡不着啊 | 
