问题
选择题
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
答案：C
考查固定句型主语从句。It is+形容词+to do sth.“做某事是......的。”_______ is difficult作主语，主语从句缺少主语，用what。故选C。