英语特指的某一天早上用on还是in?

题目

英语特指的某一天早上用on还是in?
原题：our school usually have school trip （ ）morning of April.A.in B.on C.at

答案

具体某一天用on.本题有点错误,改正后如下:Our school usually has a school trip （ on）a morning of April.A.in B.on C.at