用带on的短语填空

用带on的短语填空
1.While the guard _______,he heard a shot.
2.I’ve changed my mind.________ I'll go by plane instead.
3.I think you dropped your handkerchief _________,so that he could pick it up.
4.You must be exhausted!Did you come all that way _______?You should have taken a taxi.
5.I’m not always pleased with his work,but _______ it is satisfactory.
6.You must not,________,leave this room.
7.______,250,000 tourists visit this town each year.