A
你可以对比一下下面三个句子
That he was late was because he got up late.
That he got up late was why he was late.
(The reason)Why he was late was that he got up late.
陈述一件事情的原因有两种表达方式：
1.某某事情的原因是……
2.某某事情是因为……
原因与因为不能重复使用,这样相当于两个句子杂糅在一起了.