Question

---Must I do some washing?

---Yes, you ________, but you ________ do it at once.

1. A.
   must; mustn't
2. B.
   need; mustn't
3. C.
   must; needn't
4. D.
   need; needn't

Answer 1

C
must的否定回答用needn't，肯定回答用must。