GMAT GWD SC...為什么可以用完成時(shí)?
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,為什么不能選B?)