The reason why they failed the test was they didn’t sleep at all the night before.
A. because
B. why
C. that
D. for which
网友回答
A. because
B. why
(答案→)C. that
D. for which
解析：考查名词性从句。表语从句中不缺少任何成分，故用关系词that。The reason why … is/was …,……的原因是……。句意：他们考试不及格的原因是他们前一天晚上根本没睡觉。根据句意说明C正确。