An electron of mass $m$ is revolving around the nucleus in a circular orbit of radius $r$ and has angular momentum $L$. The magnetic field produced by the electron at the centre of the orbit is ($e = \text{electric charge}$, $\mu_{0} = \text{permeability of free space}$)

• A
  $\frac{\mu_{0} eL}{4 \pi m r^{2}}$
• B
  $\frac{\mu_{0} eL}{4 \pi m r^{3}}$
• C
  $\frac{\mu_{0} eL}{2 \pi m r^{2}}$
• D
  $\frac{\mu_{0} eL}{2 \pi m r^{3}}$