TT GWD 5-31

xiaobenzhu

    

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.

    

 

    

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

The answer is B. I agree with the answer but I don't understand the part "namely that they vary considerably"-- what 's the function in the sentence?
Thanks.

xiaobenzhu

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and can sb. help?