用带on的短语填空快!
While the guard was _____,he heard a shot.
You must not _____,lerve this room.
可以选用以下短语：on display,on show,on business,on foot,on duty,on the whole,on fire,on purpose,on the average,on any account,on second thoughts.
还有一个！You must be exhausted!Did you come all that way ______?You should have taken a taxi.