【名词&注释】
过山车(roller coaster)、休闲活动(leisure activities)、机械能不变
[单选题]过山车是一项非常惊险刺激的娱乐休闲活动．过山车时而从轨道的最低端上升到最高端，时而从最高端飞驰而下(图3)．不计能量损失，下列说法正确的是
A. a点机械能小于b点机械能
B. d点动能最大，b点势能最大，c点机械能最大
C. 由a到b的过程中，动能减少，势能增大，机械能不变
D. 由b到c的过程中，动能增大，势能减少，机械能变大
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举一反三：
[单选题]20．
A. other
B. others
C. another
D. else
[单选题]AMan invented a lot of sports equipment． The first true piece was the ball．
A. But a badly thrown stone could hurt a child． Looking for something less dangerous to throw，the peo-ple in Egypt made the first balls．
B. They used vines(藤) to keep the leaves to-gether． Later they began to use pieces of animals' skin and filled them with some light things．These balls were much better than the old ones．
C. They began to play these ball games according to the rules． They thought that ball playing was one of the best ways to teach the young men to be brave and strong．
D. The Egyptians invented _________very long ago．
E. many different games with balls
F. throwing stones
G. dangerous games
H. many different kinds of games
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