GMAT GWD SC...为什么可以用完成时?

题目

GMAT GWD SC...为什么可以用完成时?
38. GWD17-Q38:
All Y chromosomes in existence today are descended from a single ancestor’s who is thought to have lived about 140,000 years ago.
A. a single ancestor’s who is thought to have lived
B. a single ancestor’s who was thought to live
C. a single ancestor whom it is thought had lived
D. the Y chromosome of a single ancestor who is thought to have lived
E. the Y chromosome of a single ancestor whom it was thought had lived
求解答!(答案是D,为什么不能选B?)

答案

hehe,好怀念~我们用排除法做题吧...ancestor's who...指代不清,排除a.b....who was...时态错误,既然前面说...today are descended...说明这东西仍旧 is thought to...排除b.e下一个错误是...whom it...造成从句结构...