如图所示，两个用相同导线制成的不闭合环A和B，半径r_A=2r_B，两环缺口间用电阻不计的导线连接．当一均匀变化的匀强磁场只垂直穿过A环时，a、b两点间的电势差为U．若让这一均匀变化的匀强磁场只穿过B环，则a、b两点间的电势差为____．

根据句意及汉语提示，完成句中所缺单词。

      Traffic lights are very important for us. I think you know this. But do you know how they were born?
      Garrett A. Morgan was the father of traffic lights. He was an American. His family was very poor. He left school at the age of 14. He was clever. He always looked for better ways to do things. And he liked to make new things.
      At that time, every American wanted to have a car. The streets were full of cars. The traffic was really bad. “Why not put three lights at each corner ? They should be red_,green_,and yellow，” Morgan said , “ Let them tell cars to go or stop.” He made a timer (定时器)to change the lights.
     So we have traffic lights today .

如果把棱长分别为3.51cm, 2.26cm的两个正方体铁块熔化，制成一个大的正方体铁块，那么这个大正方体铁块的棱长是多少？（用一个式子表示，并用计算器进行计算，最后结果保留2个有效数字）

将欧姆定律的公式I=U/R变形得到R=U/I，关于此式物理意义，下面说法中正确的是（    ）