问题补充：
Children are having a good time in the park.
A. have fun
B. enjoy themselves
C. having fun
D. playing games
答案：
A. have fun
B. enjoy themselves
(答案→)C. having fun
D. playing games
解析：句意：孩子们在公园玩的很愉快。having a good time意为“玩的开心”enjoy themselves ， having fun 都有玩的开心的意思，但是句中有are,要用现在进行时的形式，故选C
