We arrived home at 9:00 in the evening, most people had already had supper.
A. at that time
B. by that time
C. at which time
D. by which time
网友回答
A. at that time
B. by that time
C. at which time
(答案→)D. by which time
解析：考查非限制性定语从句，由had already had 可推断出介词使用by。