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.

1. a single ancestor’s who is thought to have lived
2. a single ancestor’s who was thought to live
3. a single ancestor whom it is thought had lived
4. the Y chromosome of a single ancestor who is thought to have lived
5. the Y chromosome of a single ancestor whom it was thought had lived

为什么答案是D，请帮助。谢谢。

这道题说的是今天的Y染色体是以前一个祖宗的染色体的后代，而那个祖宗生存的年代是about 140,000 years ago. A跟B说的都是以前祖宗的染色体lived about 140,000 years ago，C说的是Y染色体是以前的祖宗，所以这三个都不对。E的whom it was啰唆，且it不知道指谁。相对之下D就非常清楚了.