impressed me was the boy’s ability to deal with this kind of difficult physics problem.
A. Which
B. What
C. It
D. That
网友回答
A. Which
(答案→)B. What
C. It
D. That
解析：考查主语从句引导词的区别。A哪一个；B 所……的=the thing that/the things that; C it 是形式主语句型为：it is +adj.+to do sth.; D that 引导主语从句不作任何成分，也没有意义。句意：所使我印象深刻的是哪个男孩处理各种物理难题的能力。故B正确。