GWD Set 6 T31

这道题其实没有比较深入地讨论过。请教NN B选项中的namely that they vary considerably中的that在这里起什么作用？namely的用法有什么讲究吗？  谢谢。

Q31:

The fact of some fraternal twins resembling each other greatly and others looking quite dissimilar highlights an interesting and often overlooked feature of fraternal-twin pairs, namely they vary considerably on a spectrum of genetic relatedness.

From dictionary:

used when saying the names of the people or things you are referring to

Three students were mentioned, namely John, Sarah and Sylvia.

[QUOTE]That some fraternal twins resemble each other greatly while others look quite dissimilar highlights an interesting and often overlooked feature of fraternal-twin pairs, namely that they vary considerably

这道题的主干参看黄色突出的部分，第一个that引导主语从句，第二个that对应于第一个that。而some fraternal twins resemble each other greatly while others look quite dissimilar，这里的look对应于resemble，都是动词，中间意思转折用了while；一些孪生子彼此极其相像而另外一些看起来十分不同。

namely that they vary considerably中的they又指代什么呢？是fraternal-twin pairs 吗？前面还有fraternal twins和others两个复数名词，有没有指代不清的嫌疑？

顶一下.

我也不知道A错在哪? 烦请牛人讲解一下好吗?

1. The fact of some fraternal twins resembling each other greatly and others looking quite dissimilar highlights an interesting and often overlooked feature of fraternal-twin pairs, namely they vary considerably

A has two problems.  and 没有while好，while 有对比的意味；namely＝ that is to say , along with n. not clause