先来分析这个句子的成分：the reason 和I am so late 不是定语关系,而是同位语的关系,因为I am so late 是 reason的具体内容.所以I am so late 前面应该用that来引导同位语从句I am so late.但是关系词that在非正式文体中可以省略.例如：He grabbed his suitcase and gave the impression he was borading the Tokyo plane.(同位语从句he was borading the tykyo plane 省去了关系词 that）.所以第一个空格可以填×.
第二空填由that引导的表语从句I missed the bus.Why和because不能连用,因为它们的意思重复,两个词只能单独使用.
因此答案选 B.×/that I missed