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
网友回答
A. it, it
B. what, what
(答案→)C. it, what
D. what, it
解析：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。