Our teacher is always busy preparing lessons until 11 o’clock at night, we students have

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

A. that time(答案→)B. by which timeC. by that timeD. which time解析：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’。

这个问题的回答的对