Complete the sentences. Choose from

Ex3. Complete the sentences. Choose from:
could have come             could have been                   could sleep               could be
1. a: Are you tired?
b: Yes, very tired. I feel as if I _ for a week.
2. a: I spent a very boring evening at home yesterday.
b: Why did you stay at home? You out with us.
3. a: Shall I open this letter?
b: Yes. It important.
4. a: How was your exam? Was it difficult?
b: It wasn’t so bad. It worse.