upon his arrival at

Question

急.高一英语句子转换题!
Upon his arrival at the office,he realized he had left his mobile phone at home.
Immediately____ _____ the office,he realized he had left his mobile phone at home.
两空到底怎么填啊?为什么呢,这里Immediately 是作连词的,一.就.,但后面加什么呢,arriving arrived 为什么呢,immediately作连词的用法?

Answer 1

他一到办公室,他意识到他已经离开了他的手机落在家里了.
_立即___ _____办公室时,他意识到他已经离开了他的手机落在家里了