must / mustn't 的用法 — 英语词汇题
题目
选词填空：根据句意和括号中的中文提示，用 must 或 mustn't 补全句子。At the train station, you ______ (必须) keep your ticket with you, and you ______ (不可以) smoke in the waiting room. When the train is moving, you ______ (不可以) open the door. But before you get off, you ______ (必须) wait until the train stops.