问题
选择题
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
答案：B
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’。