We see the lightning____it happens,but we hear thet hunder later.
20. In the past ten years, great changes _______ in my hometown.
21. The third and last chapter _______ by Professor Chen.
22. --- No, I’m afraid he isn’t in. This is his secretary speaking. Can I help you? --- _______.
23. _______ the plane have taken off on time? --- I’m afraid not. The fog was too thick then.
24. The car ran into a crowd of middle school students, _______ to hospital immediately.