Fill the gaps with in, on or at

1. in, at

2. at, in

3. at, in

4. in

5. in

6. at

7. at

8. at

9. in

10. in, at

11. at

12. at

13. in

14. in

15. in

16. at

17. on

18. at

19. in

20. on

18.