from the Madagascar periwinkle, which has proved useful in decreasing mortality

sevengu

.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,

OA is E

请问E    中 a derivative of which has proved useful in decreasing mortality among young leukemia patients是同位语（derivative指代M）还是which指代M
感觉像同为  不过解是里貌似不这么说
另外我选c  我认为原句本来说a substance iscultivated 但e把它给改了