1. A flower shop is a shop where people buy flowers.
  2. A flower shop is a shop where people buy a flower.
  3. A flower shop is a shop that people buy flowers.
  4. A flower shop is a shop people buy flowers. 5. A flower shop is a shop people buy some flowers.

------------------------ #1 is correct. What about the others? In the exam, are they regarded as correct answers or not?