Ответ:

  1. Be careful! There are lots of pickpockets in this area.
  2. 20,000 Leagues Under the Sea is an amazing book!
  3. Hide in the bushes before the angry dog sees you!
  4. Did Columbus discover America?
  5. Someone’s nephew is the son of their sister or brother.
  6. Did the children explore the old house at the end of the street?

The unused word is professor.