题目

Mary will never forget the first time she saw him. He suddenly appeared in the class one day,    61．   (wear) sun glasses. He walked in as if he   62．  (buy) the school. And the word quickly got around that he was from New York City. For some reason he sat beside Mary. Mary felt    63．  (please), because there were many empty seats in the room. But she quickly realized that it wasn’t her, it was probably the fact that she sat in  64．     last row. ____65．    he thought he could escape attention by sitting at the back, he was wrong. It might have made it用如图所示的实验装置测量杠杆的机械效率．实验时，竖直向上匀速拉动弹簧测力计，使挂在较长杠杆下面钩码缓缓上升．实验中，将杠杆拉至图中虚线位置测力计的示数F= N，钩码总重G为1.0N，钩码上升高度h为0.1m，测力计移动距离s为0.3m，则杠杆的机械效率为  %．请写出使用该杠杆做额外功的一个原因： ．