1. Everyone knows that _______ is dangerous to play with fire, but _______ is difficult is to prevent children from playing with fire.
A. it, it B. what, what C. it, what D. what, it
【陷阱】很容易误选B，认为两空均考查形式主语。
【分析】最佳答案是C，第一空填形式主语，真正的主语是其后的不定式 to play with fire。第二空填 what，what is difficult 是主语从句，注意 what is difficult 后的谓语动词 is。请做以下类似试题：
