With a relative pronoun or relative adverb:The reason ______ I d

With a relative pronoun or relative adverb:
The reason ______ I didn't attend the party was that I was notified too late.
【正确答案】：why
【题目解析】：此为why引导的限制性关系从句修饰表示原因的先行词reason，并且关系副词在从句中做原因状语。