题目：
单选题
When you want to leave your host's home, you, as a guest, can say “( )”
- AI must leave now, I have something important to do.
- BI'm afraid I must go now, it is too late.
- CI'm afraid I must be leaving now, for I have something important to do.
- DI must be leaving now, for I have something important to do.
答案：
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