大全33的新问题 ：主 + “句子” + 系表 ？？

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）

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

这个问题已有一些讨论，答案是E.

按句意等判断，无疑只有E正确 --- 但E中 a derivative of which has provide xxxx 插在主语和系表结构之间是什么？又不像独立主格？或者还原为 which has a derivative provded useful xxx ??

高人指教啊！多谢！

同位语

当初就漏看了划线句后的is cultivated导致做错