题目

一块N型半导体薄片（称霍尔元件），其横载面为矩形，体积为b×c×d，如图所示。已知其单位体积内的电子数为n、电阻率为ρ、电子电荷量e.将此元件放在匀强磁场中，磁场方向沿Z轴方向，并通有沿x轴方向的电流I。 此元件的CC/两个侧面中，________面电势高。 答案：C/          When I was seven years old, my family made me an ant farm. First , we put clean sand in a thin glass box. Then we waited for the ants to arrive. After the ants were in the glass farm, they started to make tunnels(隧道). I was amazed that each one knew exactly what to do. Each had its own job. On the fifth day a tragedy happened. I put my face so close to the glass farm that I knocked it over. All the tunnels fell down. Although the ants remained alive after their earthquake, one by one they began to die. I was scared as I watched them give up building their tunnels to carry the bodies to a corner of the farm . My mother said that the ants were dying of sadness . They simply could not stand that their tunnels were gone. Although much time has passed , I still think of that ant farm. Mom had hoped it would teach me about the natural world , but it taught me much more. Over the years, I came to realize the importance of teamwork . Working together , the ants were able to make an amazing world for themselves . I also learned that they should be admired for their hard word. But there was an even larger lesson that I did not realize until recently: adversity is a natural part of life , and must be accepted . Unlike the ants ,we cannot give up when we are sad . We have to realize that if a tunnel is gone , we must building another . Giving up , I say , is not a good choice. 1.The writer’s family made him an ant farm when he was___. A．seven years old                         B．seventeen years old. C．seventy years old                       D．six years old 2.Which statement is TRUE according to the passage? A．The writer broke the glass box because it made his face dirty. B．The ants died one by one because the tunnels were destroyed. C．Unlike people , teamwork is not so important for the ants. D．Giving up is a good way when we meet trouble. 3.The underlined word “teamwork” in the passage means_______. A小组工作     B团队合作     C辛勤劳作     D不停工作 4.What did the writer come to realize a few years later? A．Working together is useless.               B．Teamwork is important. C．He can’t accept adversity.                D．Working alone is amazing.