Here are the correct answers:

1. **at** - We are studying **at** Sai Gon University.
2. **in** - There are 40 students **in** the class.
3. **on** - I saw a mouse **on** the chair.
4. **on** - I don't want to disturb her now. She is **on** bed.
5. **on** - Mary is lying **on** the grass reading a book.
6. **in** - Is there anything interesting **in** the paper today?
7. **on** - I can’t remember the old woman walking **on** the road **at** the zebracrossing.
8. **in front of** - Don’t stand **in front of** the television. I can’t see!
9. **at** - Max had an operation. He’s still **at** the hospital.
10. **in** - Oh no! I left my wallet **in** the taxi!
11. **to** - Did you go **to** the store, yet?
12. **in** - There are a lot of magnets **in** the fridge.
13. **at** - Can we meet **at** the corner of the street?
14. **in** - There is a big tree **in** the middle of the garden.
15. **on** - My friend is **on** the way to Moscow.