写出下列单词的复数形式。

1. dog ___________________

2. puppy ___________________

3. lion ___________________

4. baby ___________________

5. cat ___________________

6. piglet ___________________

7. horse ___________________

8. calf ___________________

9. goose ___________________

10. joey ___________________

11. duckling ___________________

12. hen ___________________

13. mouse ___________________

14. child ___________________

15. knife ___________________