Electron of mass $m$ and charge $q$ is travelling with a speed along a circular path of radius $r$ at right angles to a uniform magnetic field of intensity $B$. If the speed of the electron is doubled and the magnetic field is halved the resulting path would have a radius
  • A$2\,r$
  • B$4\,r$
  • C$\frac {r}{4}$
  • D$\frac {r}{2}$
AIIMS 2009, Medium
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