选择until还是unless

until表"直到",强调时间的延续,引导时间状语从句;unless表"除非",强调条件,引导条件状语从句.看下面这两个例句：
1. I didn't know the news until my teacher told it to me.
2. I couldn't know the news unless my teacher told it to me.
句1动词didn't know表明该句强调时间的延续,表"我不知道这消息"的状态一直延续到"老师告诉我",因此应用until引导时间状语从句.
句2动词couldn't know表明该句强调条件,表"我不可能知道这消息,除非老师告诉我",因此应用unless引导条件状语从句.
本题选A,“不能下车”的这个状态一直延续到“车停下来”.
如果把本题改成：You mustn't get off the bus,则用 unless it stops,强调条件.