问题补充：
The reason he explained to us was he overslept.
A. why; that
B. why; because
C. that; because
D. which; that
答案：
A. why; that
B. why; because
C. that; because
(答案→)D. which; that
解析：考查定语从句及固定句型。Which 指代先行词the reason,在定语从句中做explain的宾语。第二空是一个句型The reason is that…原因是…;句意：他向我们解释的原因是他睡过头了。
