Ответ:

  1. Mary — she.
  2. A brother and father — they.
  3. Tom — he.
  4. Men — they.
  5. A cat — it.
  6. A schoolboy — he.
  7. A woman — she.
  8. A dog and a cat — they.
  9. My friend and I — we.
  10. An ape — it.
  11. My uncle — he.
  12. My aunt — she.
  13. Nick and Mike — they.
  14. A schoolgirl — she.
  15. An elephant — it.
  16. Grandfather — he.
  17. A woman — she.
  18. Grandmother — she.
  19. Birds — they.
  20. A lion — it.