问题补充:
I arrived home at midnight, my parents had been waiting for nearly three hours.
A. at that time
B. by that time
C. at which time
D. by which time
答案:
A. at that time
B. by that time
C. at which time
(答案→)D. by which time
解析:这题考查介词和非限制性定语从句,从句式结构看这是非限制性定语从句,又根据从句的时态:过去完成时,可以判断是by,所以填by which time。选D。句意是:我午夜到家,到那时为止我父母已经等了近3小时了。