Modal verbs (must, can't, may/might)

Change the sentences using modal verbs, then press "Check" to check your answers.
1. I’m sure she is at school.
She at school.
2. I’m sure Susan has paid the bill.
Susan the bill.
3. Perhaps he was ill.
He ill.
4. It’s impossible that they missed the plane.
They the plane.
5. I’m certain he isn't working at the library.
He at the library.
6. I’m sure she knows him well.
She him well.
7. Perhaps they went home.
They home.
8. It’s impossible that he’s telling the truth.
He the truth.
9. It is possible that he’s working late tonight.
He late tonight.
10. Perhaps they have missed the bus.
They the buss.
11. I’m certain he was sleeping.
He .
12. Perhaps she has been lying.
She .
13. It’s impossible that she saw us.
She us.
14. I’m sure they are studying.
They .
15. It is possible that she has finished her work.
She her work.