i entering the room.i found Mary ____in the desk and ___a letterA seating writing B seated writing C sitting written,

老师说用seated 和sitting都行,就是不能用 seating
entering the room.i found Mary ____in the desk and ___a letter

A seating writing B seated writing C sitting written
,老师说用seated 和sitting都行,就是不能用 seating

因为seat做动词时是及物动词,而sit是不及物动词.
seat做动词时,意思是“使某人就坐/坐下”,结构是seat sb；
而sit坐动词时,意思是“坐下”,结构是sb sit.
2. 也就是说,在此题的固定结构“感官动词found + 宾语Mary + 宾语补足语（seat/sit）”中,需要考察宾语Mary和宾补动词seat/sit之间的关系.
seat sb即seat Mary,也就是说Mary和动词seat之间为动宾关系,倒过来Mary和seat之间为被动关系,所以用过去分词seated；
而如果用sit,则Mary和sit之间明显是主动关系,所以用现在分词sitting.
但是seating是绝对不能用的.
3. 同理,Mary和write之间也是主动关系,所以后面也用现在分词writing.