The reason______he was late was ___

eason why + 句子 why+句子是定语从句
reason for + 词或短语
表示“...的原因”
上面两个做主语时,表语不得用because.
可以把 the reason (why .../ for ...) is/was that .当成固定结构记住.

当主语 是 the reason 或the cause 时 不能用 because 引导表语从句 所以答案是C.why;that
如果 用 because 应该是
that he was late was _because _he got up late.
主要原因 就是因为 why,because 重复

The reason (why he was late) was that he got up late.
这样很清楚吧？句子成分分析时把次要成分剔除，对分析句子可以简化很多。这里是：（他迟到的）原因是……；而不应该是，原因是因为……。Why he was late?Because he got up late.这样用because

“英语牛人团” 希望能够帮到你。...

主结构是 the reason was ——即主系表结构，that引导表语从句

the reason why .....is /was that是一个固定搭配的