impressed me was the boy’s ability to deal with this kind of difficult physics problem.A.

问题补充：

impressed me was the boy’s ability to deal with this kind of difficult physics problem.

A. Which

B. What

C. It

D. That

答案：

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。