prep 2#220 namely的用法，请进

220.
            (29238-!-item-!-188;#058&003590)     (GWD-6-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.

A. 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（the fact后面应该加同位语从句）

B. 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

C. 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（it做主语，通常指代前句中的主语，前句为分词结构，没有可指代的东西，后面没有to do / that 句型，it也不能作为形式主语）

D. 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（it 没有可以指代的对象）

E. 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（无因果关系的！）

请教B选项中红色的that是个什么东东？～～～请帮忙解答下namely的用法，谢谢～