1.So far (HTHT)
2. since 1995 (HTHT)
3. told (QKD)
4. have..heard(HTHT)
5. haven't had(HTHT)
6. have been working (HTHT)
7. today
8.recently(HTHT)
9. since June (HTHT)
10. for two hours (HTHT)
11. be held (tương lai, bị động)
12. is exported (hiện tại, bị động)
13. is heated (hiện tại, bị động)
14. shouldn't be kept (hiện tại, bị động)
15.haven't eaten (since->HTHT)
16.ten years ago
17. met (ago-> QKD)
18. have taken(since->HTHT)
19. have met
20. have ... traveled
21. is being repaired