题目内容：

It's fun ___ the boys playing basketball.A watch B to watching C watching D watches

优质解答

B
It is fun to do.
It为形式主语,真正主语为to do,就是说“to do sth”is fun.(做...是有趣的)
例句：（来自百度词典）
It may be fun to play chess.
下棋可能是有趣的.