回答：
at a meeting和in a meeting都可以說,請(qǐng)看實(shí)例(大都出自權(quán)威詞書)：
Officially,the director is in a meeting,though actually he’s playing golf.據(jù)說主任正在開會(huì),可他實(shí)際上正打高爾夫球呢.——見《牛津高階英漢雙解詞典》(第四版) (officially詞條)
I’m tied up in a meeting until 3 pm.我開會(huì)直到下午三點(diǎn)鐘方可脫身.——見《牛津高階英漢雙解詞典》(第四版) (tie詞條)
Sorry to keep you waiting—I got stuck in a meeting.——見《朗文當(dāng)代英語詞典》(1995年版) (keep詞條)
Mrs Lavelle is in a meeting at the moment.——見《朗文當(dāng)代英語詞典》(1995年版) (meeting詞條)
There were three others present at the meeting besides us.除了我們之外,還有其他三人個(gè)出席了會(huì)議.——見《朗文多功能分類詞典》(besides詞條)
He took the chair at a meeting some time ago.前些日子他主持了一次會(huì)議.——見王福禎編著《新時(shí)代英語慣用法詞典》第195頁.
There were both blacks and whites at the meeting.參加會(huì)議的有黑人也有白人.——見周貞雄編著《大學(xué)英語重點(diǎn)詞用法手冊(cè)》besides詞條)
She might be invited to speak at the meeting.她可能被邀在會(huì)上發(fā)言.——見張道真編著《張道真英語語法》第116頁.