13. In geometry, an ellipse may be defined as the locus of all points -------distances from
two fixed points is constant.
(A) which as the sum of
(B) of the sum which
(C) whose sum of whose
(D) whose sum that the
这道题有好多人问过，但是我现在依然要请教：网上的C的答案有的是 the sum of whose不知道哪个对？
如果是这个C的话，那就选C吗?A为什么不对啊？
