求助GWD-6-Q31

minigreen

220. (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
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
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        
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

答案是B，想问问D为什么不对

prep语法笔记里面对D选项的解释是：
   
It指代正确，因为是结构it is a fact that结构的一部分，强调句；a fact无所指；with独立主格的逻辑主语错误；namely无补充说明对象。

a fact无所指是什么意思？
with独立主格的逻辑主语错误是什么意思呢？独立主格结构不是自带逻辑主语吗？