A.on the morning of May 7th
指“在早上,在上午”,用 in the morning.
指具体某一天,几月几号,星期几的早上,用 on ,变成 on the morning of February 23,on the morning of last Wednesday.也可以说,当 morning,afternoon,evening 有定语修饰时用 on,如 on a rainy morning （在一个下雨天的早上）