"at the ceremony" or "on the ceremony"? He was elected to make a speech [at] the commencement ceremony as [the] representative of new students. Here are my two questions: 1. "at the ceremony" or "on the ceremony"? I'm not sure, because I was told both of them are correct. 2. as [a] representative of students or as [the]representative of students? Are both of them correct? The question about articles always makes me confused... Please give some advices~Thank you so much!!