Ex.1 Underline the correct answer

A: Good morning, madam. May/must I help you?
B: You might/mustn’t be able to. I need a ball of wool that is the same color as my jumper.
A: Just a moment, please. I must/mustn’t look in the stock room. We might/might not have some left. I’m not sure.
B: The wool must/mustn’t be exactly the same color, it must/mustn’t be different.
A: Could/Mustn’t I ask you a question, madam?
B: Yes.
A: What do you want the wool for?
B: I could/must finish knitting this jumper. It only has one sleeve at the moment!