the indefinite Artcles "a" and "an"

Write "a" or "an" if necessary
1. ____ pencil 2. ____ wall 3. ____ eggs
4. ____ clock 5. ____ desk 6. ____ umbrella
7. ____ dog 8. ____ hen 9. ____ fish
10. ____ windo 11. ____ uncle 12. ____ orange
13. ____ man 14. ____ girl 15. ____ apple
16. ____ rulers 17. ____ classroom 18. ____ boy
19. ____ duck 20. ____ picture 21. ____ box
22. ____ blackboard 23. ____ cars 24. ____ flower
25. ____ woman 26. ____ teacher 27. ____ university
28. ____ student 29. ____ shirt 30. ____ pens