–When did he come back last night?It was at 8 o’clock I was watching a documentary on

问题补充：

–When did he come back last night?

--It was at 8 o’clock I was watching a documentary on TV.

A. since

B. that

C. when

D. which

答案：

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。