I saw an old woman --under a tall tree with his hands----,th

正确答案是：seated,crossed,而不是：seat.crossed.
此题考查“感官动词saw/see + 宾语 + 宾语补足语”结构.
1.关键在于对seat做动词的理解.
seat为及物动词,意思是“使（某人）就坐”,结构是seat sb (+ 介宾）,也就是说sb是seat动作的承受者,变成被动即：sb be seated...“某人坐在...”；
所以句中宾语an old man和seat之间也是被动关系,所以必须用过去分词seated做宾语补足语.
此用法同dress,也经常说：sb be dressed in...
2.至于第二空,cross的意思是“使...交叉”,cross his hands变被动即his hands be crossed,说明宾语his hands和cross之间也是被动关系,所以cross作宾语补足语时,也用过去分词crossed.