以下是引用hhiconnie在2008-9-18 7:46:00的发言：
21-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 single ancestor’s who is thought to have lived
- a single ancestor’s who was thought to live
- a single ancestor whom it is thought had lived
- the Y chromosome of a single ancestor who is thought to have lived
- the Y chromosome of a single ancestor whom it was thought had lived
Answer: D 为何是to have lived ,后面的ago不是说明应该用过去时态么 用排除法： A/B， a single ancestor's + who， 所有格后面接定语从句引导词who错，应该是ancestor+who; C/E， 用了whom，可是whom只有在介词后才用，所以错， 于是只有D，不过当时做题时没有想你提的问题，  | 
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发表于 2008-9-18 07:46:00
