题干
—Why was John late for school this morning?
—I don't know the _______. Maybe you can ask his good friend Simon.
A：opinion
B：problem
C：reason
答案(点此获取答案解析）
C
和
粒子氦核42He以相同的速率在同一匀强磁场中做匀速圆周运动，轨道半径分别为
和
，周期分别为
和
，则下列选项正确的是（ ）。
