Entering the reading room,I found Mary ---at a desk,her atte

C 1.sit与seat的用法区别：sit为不及物动词,所以没有被动语态；seat为及物动词,用法应为sb be seated或sb seat oneself.所以本题的应接sitting或seated来做非谓语.选项中没有sitting,故选seated.2.fix的用法搭配：sb.fix one's attention on sth,表示被动时是one's attention is/was fixed on sth.
这里应该用被动,即one's attention is/was fixed on sth.实际上,本题中“her attention”前省略了“with”,表示伴随着.而前面有了“with”,后面不用be动词.所以,应该为“(with) her attention fixed on a magazine”.故选“fixed”.