Can you tell me who spoke _ the meeting

题目

Can you tell me who spoke _ the meeting
A:at B:on C:in D:by
答案给的是A,为什么,其他答案有什么不对,为什么
on the meeting/in the meeting/at the meeting 都有什么区别,都有没有语法错误

答案

in the meeting 出席会议.
i'll be in the meeting.
at the meeting 在会议上.
其他的没有见过.