题目

  My mother once asked me what the most important part of the body was. When I was   36  , I thought music and talking were the most important things, so I said, “My   37  , Mommy.” She said, “No. Many people are deaf (聋的) and they can’t hear anything but they live usual lives. Please   38   thinking about my questions and I will ask you again soon.”  A few years passed before she asked me again. This time I told her, “Mommy,   39   is very important to everybody, so it must be our eyes.” She looked at me and said, “You are learn一带电粒子以初速度v从a点进入匀强磁场，如图，运动中经过b点，Oa=Ob=x，若磁场的磁感应强度为B，则粒子在磁场中运行的轨道半径为（ ）A．B．C．D．