Ответ:

  1. deer — deer
  2. species — species
  3. fish — fish
  4. bison — bison
  5. sheep — sheep
  6. trout — trout
  7. salmon — salmon
  8. wolf — wolves
  9. leaf — leaves
  10. life — lives
  11. shelf — shelves
  12. wife — wives
  13. half — halves
  14. scarf — scarves
  15. mouse — mice
  16. foot — feet
  17. tooth — teeth
  18. man — men
  19. child — children
  20. goose — geese
  21. woman — women