题目

听下面一段较长对话，回答问题。 （1） How long did the boy spend his holiday on a small hill? A . For 25 days. B . For 30 days. C . For 35 days. （2） How did the boy get his food and water? A . From the farmers. B . From his friends. C . By himself. 答案：B C在杂技表演中，有一项跳板节目，若支点在跳板的中央，当质量为60千克的两名男演员同时从距离地面3米高处由静止下落到跳板的一端时，静立于另一端的质量为40千克的一名女演员被向上弹起4米高．若不考虑空气阻力和人的内力做的功．则跳板的机械效率为（ ）A．88.9%B．85.3%C．44.4%D．40.4%