题目

（20分）如图所示，两足够长平行光滑的金属导轨MN、PQ相距为L，导轨平面与水平面夹角a=30°，导轨电阻不计．磁感应强度为B的匀强磁场垂直导轨平面向上，长为L的金属棒ab垂直于MN、PQ放置在导轨上，且始终与导轨接触良好，金属棒的质量为m、电阻为R．两金属导轨的上端连接右端电路，灯泡的电阻RL=4R，定值电阻R1=2R，电阻箱电阻调到使R2=12R，重力加速度为g，现将金属棒由静止释放，试求： （1）金属棒下滑的最大速度为多少？ （2）R2为何值时，其阅读理解     The common cold is the world’s most widespread illness, which is plagues(疫病) that flesh receives.     The most widespread fallacy(谬误) of all is that colds are caused by cold. They are not. They are caused by viruses passing on from person to person. You catch a cold by coming into contact, directly or indirectly, with someone who already has one. If cold causes colds, it would be reasonable to expect the Eskimos to suffer from them forever. But they do not. And in isolated arctic regions explorers have reported being free from colds until coming into contact again with infected people from the outside world by way of packages and mail dropped from airplanes.      During the First World War soldiers who spent long periods in the trenches(战壕), cold and wet, showed no increased tendency to catch colds.     In the Second World War prisoners at the notorious Auschwitz concentration camp(奥斯维辛集中营), naked and starving, were astonished to find that they seldom had colds. At the Common Cold Research Unit in England, volunteers took part in Experiments in which they gave themselves to the discomforts of being cold and wet for long stretches of time. After taking hot baths, they put on bathing suits, allowed themselves to be with cold water, and then stood about dripping wet in drafty room. Some wore wet socks all day while others exercised in the rain until close to exhaustion. Not one of the volunteers came down with a cold unless a cold virus was actually dropped in his nose.      If, then, cold and wet have nothing to do with catching colds, why are they more frequent in the winter？Despite the most pains-taking research, no one has yet found the answer. One explanation offered by scientists is that people tend to stay together indoors more in cold weather than at other times, and this makes it easier for cold viruses to be passed on.      No one has yet found a cure for the cold. There are drugs and pain suppressors(止痛片) such as aspirin, but all they do is relieve the symptoms. 1. The writer offered _______ examples to support his argument.A. 4 B. 5 C. 6 D. 32. Which of the following does not agree with the chosen passage?A. The Eskimos do not suffer from colds all the time.B. Colds are not caused by cold. C. People suffer from colds just because they like to stay indoors.D. A person may catch a cold by touching someone who already has one.3. Arctic explorers may catch colds when _______.A. they are working in the isolated arctic regionsB. they are writing reports in terribly cold weatherC. they are free from work in the isolated arctic regionsD. they are coming into touch again with the outside world4. Volunteers taking part in the experiments in the Common Cold Research Unit _______.A. suffered a lot  B. never caught coldsC. often caught colds D. became very strong5. The passage mainly discusses _______.A. the experiments on the common coldB. the fallacy about the common coldC. the reason and the way people catch coldsD. the continued spread of common colds