题目

With a hat by his feet，a blind boy sat on the steps of a building. He held up a sign which said， I ' m blind, please help. Only a few people put    16    in the hat. A man was walking by. He    17    some coins into the hat. He then turned the sign    18  . wrote some words, put the sign back and left.Soon the hat began to fill up. A lot more people   19    to give money to the blind boy. That afternoon the man came to see    20    things were. The boy recognized his footsteps (脚步声) and asked, Were you the one who    21    my sign this morning? What （本小题8分）已知且，求的最小值