Here are the appropriate words to fill in the blanks based on the context:

18. A: Is this your CD?
B: No, it isn't mine. **I think** it **belongs** to Peter.

19. A: Why can't you wash your dirty plates sometimes? **Do you** leave them in the sink most of the time?
B: OK, sorry. The last few weeks **I've had** so little time. **I rush** around all the time.

20. A: I've decided to try and learn a foreign language.
B: Have you? Which language **are you going to learn**?
A: Spanish.
B: When **does the Spanish course start**?
A: **It starts** next week.
B: That's great. I'm sure **you'll enjoy** it.
A: I hope so. But I think **it'll be** quite difficult.

21. A: **I'm having** a party next Saturday. Can you come?
B: On Saturday? I'm not sure. Some friends of mine **are coming** to stay with me next week but I think **they'll go** by Saturday. But if **they're** still here, **I might not be** able to come to the party.
A: OK. Well, tell me as soon as **you know**.
B: Right. **I'll phone** you during the week.