Question

Is that the reason_____he was late?

A that Bwhich C why为什么选C不选AB

Answer 1

该从句是定语从句。

定语从句里，that和which都是关系代词，关系代词必须充当从句的一个名词性成分，比如把原句改一下,Is that the reason ________he told you? 如果是这句，就用that和which了，因为that和which做从句里you后面的第二个宾语。而现在这句话，he was late完整，不缺少名词性成分，所以只能用关系副词why引导。记住了，定语从句里，why，when，where这三个是关系副词，引导的定语从句都必须是完整的，不缺名词性成分。

Answer 2

原因状语从句，选A误以为是定语从句。