Don't come into the lab ______ you are asked to. A. unless B. until C. because D. since 答案是A还是B, 感觉两个都可以，麻烦还是解释下。
not...unti..和not..unless
解题思路： 此题属于连词的考查，根据连词意思及句意进行解答
解题过程：
A
【解析】句意：不要进入实验室除非你被叫进去。A除非；B直到；C因为；D既然。此题考查连词，根据句意，故选A。
考点：考查连词。
作为从属连词含义不同,引导从句性质不同unless与until均为从属连词,unless表“除非”,强调条件,引导条件状语从句; until表“直到”,强调时间的延续,引导时间状语从句。
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.
3.—Was his father very strict with him when he was at school?
—Yes. He had never praised him until he became one of the top students in his grade.
析: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。
句3表明“他爸爸从来没有表扬他”的状态一直延续到“他变成年级最好的学生之一”,因此也应用until引导时间状语从句。
最终答案：A
