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
答案是C
37. Wampum, beads used as [a form](A) of exchange [by](B) some Native Americans, was made of [bits of](C) seashells cut, [dril](D)l, and strung into belts.