Respuestas
When completing the dialog regarding Anna and Lucy are camping in a field with could/may/might/must or can't results:
Lucy: Anna, Anna, wake up!
Anna: What's the matter? It CAN'T be time to get up. It's still dark.
Lucy: I heard a strange noise. There's someone or something outside the tent. Can't you hear it?
Anna: I can't hear anything. You MAY be dreaming, go back to sleep!
Lucy: I'm sure I heard something, I suppose it MIGHT be a cow or sheep or something, but there weren't any animals in the field when we put up the tent. I'm going outside to check. Anna, look, there's a light over there by the trees.
Anna: There CAN'T be! We're in the middle of the country and there aren't many buildings in sight.
Lucy: Well, what's that, then?
Anna: I don't know! It MUST be a car or a motorcycle, or something like that.
Lucy: But the road's over there on the other side. And can you hear a car or a motorcycle?
Anna: No, but what else COULD it be?
Lucy: Strange noises and lights... Well it MIGHT be visitors from another planet.
Anna: No, it CAN'T be. There is no life on other planets! It's probably the farmer. He MAY be checking his cows.
Lucy: Yes, but what if it isn't?