20. C. for 2 hours ( have been waiting for you )
21. D. had left ( by the time )
22. C. has written ( by the age of 25)
23. B. wrote
24. C. had broken down ( a few days previously )
25. C. have tried ( the first time )
26. B. he left (since)
27. B. had had ( after )
28. A. will have learned (by the time )
29. B. walked
30. A. was going (when)