the reason -- he was late for the meeting was -- he missed t

答案：填why,that
即：The reason why he was late for the meeting was that he missed the first bus this morning.
解析：第一个空是why引导的定语从句,修饰前面的the reason,第二个空是that引导的表语从句
翻译：他开会迟到的原因是今天早晨他错过了头班车.
有不明白的请继续追问,
再问： 请问有没有the reason is because/why 的结构的？
再答： 大家都知道中国老师教的都是the reason why...is后面一定要接that引导表语从句，但是 参考美国传统词典because词条的特别说明： Usage Note: A traditional rule holds that the construction "the reason is because" is redundant, and should be avoided in favor of "the reason is that." The usage is well established, however, and can be justified by analogy to constructions such as "His purpose in calling her was so that she would be forewarned of the change in schedule" or "The last time I saw her was when she was leaving for college". All three constructions are somewhat less than graceful, however. 可见， is because 的用法已经摆脱了语法的束缚，成为既成事实了。
再问： 那也就是在语法课中可以使用咯
再答： 应该可以，不过，如果给学生讲课，最好还是遵循我们中国老师上课的习惯吧，否则会引起歧义，呵呵