核心问题
- Everyone was in their room when the bell rang. (全国研考)
everyone/every one
- Everyone is somewhat touchy about their home and personal property, they are too private to be exposed to the inspection of strangers. (《英语考试题典》, 吉林科技出版社, p. 455)
上面两道试题要求将 their 改为 his。我认为，这里用 their 并没有什么错误。
词典解答
正像语言学家们所指出的, everyone 具有整体意义, 因此与之有关的代词可采用复数形式:
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Everyone thinks they have a right to be here. (CGEL) 所有的人都认为他们有权在这里。
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Everyone has arrived, haven't they? (CGEL) 大家都来了,不是吗?
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Everyone has their own ideas about it. (EED) 关于这件事每个人都有自己的想法。
更详细的情形,参见 everybody/everyone...they。