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. what; it
D. it; what
网友回答
A. it; it
B. what; what
C. what; it
(答案→)D. it; what
解析：考查it作为形式主语和主语从句。第一空的it是形式主语，真正的主语是后面的不定式to play with fire;第二空是一个主语从句what is different，在这个主语从句中what引导起主语从句且作为主语。句意：每个人都知道玩火是很危险的，但是真正困难的是阻止孩子们玩火。故D正确。