题目：
Every country had its heroes. The heroes are the people that nation and especially the young people 21_____ . If you get a list of the 22 _____of a nation, it will tell you the potential (潜力）of the nation. If today in America ’ you ask the high school students to list their heroes their 23____ would probably divide into three groups. rHie first group of heroes would be the rock stars—the people 24 _____with rock music. There is no doubt that such people do have talent (天才）but one 25_____ if one should hold up rock stars as model. The rock stars too often are mixed with drugs and their personal life isnot all that — 26 ____ . The rock stars are rich and wear the latest fashion styles (时装）.However oneshould 27 _____more in a hero than such things as 28______ and fine clothes. A second type of heroes for the American youth are sports stars. Again you have a person who has a great talent in one area_ 29 _____. However, too often the personal life of the sports star is a bit 30_____. Too frequently drugs and drinking are a part of 31_____ of the sports stars. A third group of heroes are TV or movie stars. These people may have lots of 32___ ，talent and are quite handsome. However ’ the personal life of too many actors is quite sad and should not be held up as a 33____ for young people. Today，the rock stars，the sportsmen and the actors 34____ have become the models of the youth in America. Really,do you hear a young person say that his 35____ is a doctor, a teacher or a scientist? These people are not 36____ and do not wear fashionable clothes. However, they are 37____ people who work hard to make the world a better place for everyone. What is really sad is that the young try to 38____ their heroes. They like to wear the same clothes as theirs, if the heroes of today for the American youth are 39 ____only to rock stars, sportsmen and actors, the future does not look too 40_____ .
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答案：
小题1:B
小题2:D
小题3:A
小题4:B
小题5:C
小题6:D
小题7:C
小题8:B
小题9:A
小题10:B
小题11:B
小题12:C
小题13:A
小题14:C
小题15:D
小题16:A
小题17:B
小题18:D
小题19:B
小题20:A
cm的圆形区域内，有一个水平方向的匀强磁场，磁感应强度大小为B2=0.1T，方向垂直纸面向外。M、N为竖直平行放置的相距很近的两金属板， S1、S2为M、N板上的两个小孔，且S1、S2跟O点在垂直极板的同一水平直线上。金属板M、N与一圆形金属线圈相连，线圈的匝数n=1000匝，面积S=0.2m2，线圈内存在着垂直纸面向外的匀强磁场，磁感应强度大小随时间变化的规律为B1=B0+kt（T），其中B0、k为常数。另有一水平放置的足够长的荧光屏D，O点跟荧光屏D之间的距离为H=2R。比荷为2×105 C/kg的正离子流由S1进入金属板M、N之间后，通过S2向磁场中心射去，通过磁场后落到荧光屏D上。离子的初速度、重力、空气阻力及离子之间的作用力均可忽略不计。问：
