1、on /upon doing sth. 刚一做某事（就）. 结构：介词+动名词. 功能：相当于as soon as引导的时间状语从句。. 例如：. On arriving …
They are both correct and used every day, the meaning is the same (at the event of arriving). I would say “upon” is …
Basic needs of returnees are met upon arrival. 在返回者到达后满足其基本需要。. This rental agreement must be signed …
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