We arrived home at 9:00 in the evening most people had already had supper.A. at that time

问题补充：

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

答案：

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。