头晕的从句。。。

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

为什么选a? 烦请大虾还原本句。谢过！

同问，这道题我在真题里做也是whose sum of whose 中间没有逗号隔开。但是看到语法大全里不一样。 如果没有标点符号隔开，the sum 和 all points 两个名词直接连在一起，没有问题吗？如果是这样，whose sum of whose 好像要通顺一点。