【賽魯】評論
"upon" means something is about to happen. And only "upon" can follow with a noun among these choices.
【hsing1992】評論
____大學畢業後,我去找了工作。(A) 當...的時候
【月嬋】評論
upon 一旦...,S就~本題翻譯,一旦從大學畢業,我就去找工作。