–When did he e back last night?
--It was at 8 o’clock I was watching a documentary on TV.
A. since
B. that
C. when
D. which
网友回答
A. since
B. that
(答案→)C. when
D. which
解析：考查定语从句。句中先行词为8 o’clock，在定语从句中作时间状语，故用关系副词when。句意：——昨晚他什么时候回来的？——8点钟，当时我正在看一部电视纪录片。故C正确。