Here are the best answers for each sentence:

1. D. hears
2. A. met
3. A. were learning
4. C. was watching
5. D. went
6. B. had
7. C. Was
8. B. listen
9. A. started
10. C. are watching
11. A. had finished
12. A. used to
13. C. had to
14. B. was leaving / forgetting
15. A. got / had been waiting
16. C. had written
17. C. crossed
18. A. took
19. C. were not doing / was cooking

Feel free to ask if you need any further clarification!