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

选D，楼主的问题是什么？

应该选D.

1. "be thought"是被现在的人认为，所以用is而不是was。

2.be thought/known to have done是一个好的表达，表明done的事情发生在过去，并不是表示完成。举个例子：

如果用It is known造句：It is known that Edison invented electronic light bulbs in 2000.

如果把这个句子改称be known to的形式就要用: Edison is known to have invented electronic light bulbs in 2000.

3. ABC中说现存所有“染色体”是从一个“人”繁衍下来的，是不合逻辑的。

4.C和E用whom指代ancestor是错的，因为ancestor在句子作主语，必须用who.

不太同意松鼠gg的第三条解释，AB选项和C不同，在ancestor后面加了's，所以你的第三条解释不能排除AB

我看的时候觉得是AD中间选一个，个人认为A简洁，D关系清晰，所以想了半天也不知道选哪个好，哪位牛人能解释一下为什么是D？

呵呵，我第3条是说错了，

ABC不正确，因为用的是ancestor's的's所有格形式，这样后面的关系代词who就不可以指向ancestor了。

是这样么？

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