题干
The missing children haven’t been found yet. Something terrible ________ to them.
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A：may happe B：should have happened C：must happe D：might have happened 答案(点此获取答案解析）
D
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)和a粒子(
)以相同的初速度垂直
进入同一匀强磁场做匀速圆周运动。则质子和a粒子的轨道半径之比为：
