Complete the sentences with must, might (not) or can't.

1. That's ridiculous. It ______ be a poltergeist. They don't exist!
2. Listen. I can hear Mum's voice. They _______ be back from the theatre.
3. OK. It _______ be a ghost. But until we have proof, I have my doubts.
4. Don't go down to the cellar. You ________ come back!
5. Until we're certain of the facts, we have to accept that it ________ be a practical joke.