一道语法题

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

为什么选b？the fact of 是不是不对啊？

另外excepting放在句首也不对吧

that 放在句首吗 翻译成”......,也就是说........"???