the teacher was convinced of the reason ____he was caught in a traffic jam. 为什么是that

Question

the teacher was convinced of the reason ____he was caught in a traffic jam.
为什么是that不是which？
为什么是that不是why 。句子缺少什么成分？大神求帮助！

Answer 1

that he was caught in a traffic jam.是同位语从句，所以用that追问

好的谢谢您