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