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?