这道题的选项

答案：C
分析：考查on doing sth用法,表示"一……就……"的意思。具有该用法的短语还有：as soon as,the moment,hardly/scarcely…when/before， no sooner…than.比如:”我刚进教室，上课铃就响了“，可以用下列表达：
1.On entering the classroom,the bell rang.
2.As soon as/The moment I entered the classroom,the bell rang.
3.I had hardly/scarcely entered the classroom when/before the bell rang.
4.No sooner had I entered the classroom than the bell rang.

在本题中，除了用on,还可以说：
1.As soon as/The moment the child see the damage he had done,he quickly ran away.
2.The child had hardly/scarcely see the damage he had done when/before he ran away.
3.No sooner had the child seen the damage he had done than he ran away.
你明白了么？

投C一票，这题可以用排除法的哦！by表方式，for表原因，而这里只是说当、、的时候，at也不合适，直觉！

C on,表示一见到就跑的意思，这里on作介词.

B