$A$ metal surface having work function '$W_{0}$' emits photoelectrons when photons of energy '$E$' are incident on it. The electron enters a uniform magnetic field '$B$' in a perpendicular direction and moves in a circular path of radius '$r$'. Then '$r$' is equal to (where '$m$' and '$e$' are the mass and charge of the electron,respectively).

• A
  $\frac{\sqrt{m(E-W_{0})}}{eB}$
• B
  $\frac{m(E-W_{0})}{eB}$
• C
  $\frac{\sqrt{2m(E-W_{0})}}{eB}$
• D
  $\frac{2m(E-W_{0})}{eB}$