Question

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

Answer 1

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’。