【题目】It is so cold that you can't go outside_____ fully covered in thick clothes.
A.if
B.unless
C.once
D.when

【答案】B
【解析】句意： 天太冷了，除非穿上厚衣服否则你不能出去。A. if”如果“；B. unless”除非“；C. once”一旦“；D. when”当“。根据句意故选B。