写出下列名词的复数形式：6. tomato 7. beef 8. Negro 9. girl 10. grow-up11. boy student 12. American 13. piano 14. foot 15. mouse16. fish 17. dress 18. watch 19. box 20. photo21. key 22. wife 23. leaf 24. child 25 man doctor26. Chinese 27. Englishman 28. strawberry 29. day 30.sheep31. baby 32. German 33. rice 34. milk 35.CD36. deer 37. family 38. policeman 39. son-in-law 40. daughter-in-law