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

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’。