4. C → was
5. A → had gone
6. B → had arrived
7. A → have
8. A →will have
9. B → would have reached
10. C → would
11. B → if
12. I would have offered to help him if I had realized that he was ill. (câu này mình thấy hơi lạ nên mình đánh hết nguyên câu)
13. B → doesn't start
15. C → wouldn't
16. C → had
17. C → hadn't seen
18. C → would have taken
19. A → would you do
20. B → hadn't sneezed