31. You see the lightning _____ it happens, but you hear the thunder later.
17. It’s known that taking exercise______your health.
16. He gave a ________smile.
15. They gave a banqnet( 宴会)_____the guests from Britain.
14. I’m really tired_____Tom . He had me _____for two hours in the rain .
13. If you cheat in the exam, you could hardly _____it.