Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the following questions.

I have three brother, x are professional athletes.

A. two of them

B. two of that

C. two of who

D. two of whom

Bảng tuần hoàn có bao nhiêu nhóm B? Các nhóm B gồm bao nhiêu cột?