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一正弦交流电的电压随时间变化的规律如图所示。由图可知（  ）

A．该交流电的电压瞬时值的表达式为u＝100sin(25t)V

B．该交流电的频率为25Hz

C．该交流电的电压的有效值为141.4V

D．若将该交流电压加在阻值R＝100Ω的电阻两端，则电阻消耗的功率时50W

答案

BD

不定项选择

关于移动电话叙述正确的是[ ]

A．移动电话就是固定电话

B．移动电话的工作原理与固定电话的工作原理完全一样，没有区别

C．移动电话即可以发射电磁波，又可以接收电磁波

D．移动电话只可以接收电磁波
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Many animals engage in play，but homo sapiens is the only animal (1) have invented sports.Since sports are an invention，a part of culture (2) than an aspect of nature，all definitions of sports are somewhat arbitrary. (3) sports are a human universal found in every known culture or a (4) unique to modern society depends upon one’s definition of sports.Men and (5) have always run，jumped，climbed，lifted，thrown， and wrestled，but they have (6) always performed these physical activities competitively. Although all literate societies seem to (7) contests of one sort or another in which men， and sometimes women， (8) in displays and tests of physical skill and prowess，sports may be (9) defined as physical contests performed for their own sake and not for (10) ulterior end.According to this strict definition，neither Neolithic hunters nor contestants (11) religious ceremonies such as the ancient Olympic Games were engaged in sports. (12) on the stipulation that sports must be performed for their own sake (13) the paradoxical elimination of many activities which are usually thought of as (14) ，such as exercises done for the sake of cardiovascular fitness，races run (15) satisfy a physical education requirement，ball games played to earn a paycheck. (16) definition also means abandonment of the traditional usage in which"sport，"derived (17) Middle English disporter，refers to any light hearted recreational activity.In the. (18) of some 18th- century aristocrats，a game of backgammon and the seduction of (19) milkmaid were both considered good sport，but this usage of the term (20) become archaic.
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