One morning,a blind boy sat on the steps of a building -----

and 是连词,连词必须连接两个完整的句子。而后面的 a hat next to his feet 并不是完整的句子,它缺少谓语（应该是a hat is next to his feet） 所以呀,这里不可以用and/but之类的介词,而应该用with（表伴随,大概是状语那样）