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

B. What

C. It

(答案→)D. That

解析：考查名词性从句。引导主语从句并在主语从句中做主语。句意：给我留下深刻印象的是男孩在处理这种物理难题时的能力。