题目

Our teacher is always busy preparing lessons until 11 o’clock at night, ______we students have gone to sleep. A. that time       B. by which time C. by that time     D. which time 答案：B 解析: 选B。句意为“我们的老师总是忙着备课到晚上11点钟，那时我们已经睡了”。by意为“到……时(为止)”，which time 指代 “11 o’clock at night’。

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