Question

【题目】— What were you doing ______ I knocked at the door?

— I was sleeping.

A. unless B. once

C. when D. while

Answer 1

【答案】B

【解析】试题分析：句意：－我敲门时，你在做什么？－我在睡觉。A．unless如果不，除非；引出条件状语从句；B．once一旦……，引出时间状语从句；C．when当……时；D．while当……时，后跟延续性动词。本题表示“当……时”，knock是非延续性动词，故选B。