题目

 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 答案：D 解析: 最佳答案是D，很容易误选B，认为两空均考查形式主语。 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：

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