1 A: Mustn't/Must I eat all my broccoly?
B: Yes. It's very good for you.
2 A: Must/Could I do the washing-up now?
B: Yes, because I want to make a cake afterwards.
3 A: I saw a suspicious man in our street yesterday, but I didn't call the police.
B: You ought to/need have called them immedialy.
4 A: The beach can/could get very crowded tommorow as it's a holiday.