Вставить артикли a/an и объяснить почему

1. I think we must call a doctor.
2. Do you know the Browns? They live next to us.
3. Peter is the tallest in the class.
4. There are some beautiful towns in the north of Italy.
5. Greece is in the south.
6. The President is the head of state in the US.
7. What did you have for dinner?
8. Trafalgar Square is in London.