再问GWD-6-31

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.

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
           
2. 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
           
3. With some fraternal twins resembling each other greatly and others looking quite dissimilar, it highlights an interesting and often overlooked feature of fraternal-twin pairs, namely considerable variation
           
4. With some fraternal twins resembling each other greatly and others looking quite dissimilar, it is a fact that highlights an interesting and often overlooked feature of fraternal-twin pairs, namely a considerable variation
           
5. Because some fraternal twins resemble each other greatly and others look quite dissimilar, this fact highlights an interesting and often overlooked feature of fraternal-twin pairs, namely they vary considerably

正确选项B 

请问：that在这儿的功能是什么？

搜到的唯一一个帖子上，有人说是补充状语从句。才疏学浅，我没有听过“补充状语从句”这种说法；只知道when, where, if等引导的状语从句。难道that也可以单独引导状语从句？

那位帮忙解答一下，谢谢先

我的理解是:that 是指示代詞，指的是前面提過的 feature，因為這個 feature要用句子表達，因此用 that 代替。它的功能好比下面這個句子一樣:

He posseses a very good persnality trait that he loves to help people in need.

哦，明白了，我查过了，vary可以当及物动词用，如：She varied her dress as fashion changes.

所以，我想应该是这样的： that引导的其实就是一个定语从句，即 an feature that they(twins) vary considerably on...

有不对之处，请再指出。谢谢！

此处的that为同位语引导词

这里that引导的不是定语从句。如果这里是个定语从句，则它就应该是一个非限定性定语从句，该由which来引导
同意LS和LSS的说法，namely的意思为“即, 也就是”，表示一种进一步说明的关系，所以这里的that从句应该是同位语从句

当初排除B就是因为怀疑它在考namely的用法,又好像namely后面一般接的都是名词....

难道可以用namely that表解释或换一种说法,等同于that is to say,specifically?

google到一句: