$A$ photon of energy $E$ ejects a photoelectron from a metal surface whose work function is $W_0$. If this electron enters into a uniform magnetic field of induction $B$ in a direction perpendicular to the field and describes a circular path of radius $r$,then the radius $r$ is given by,(in the usual notation)

• A
  $\frac{\sqrt{2 m(E-W_0)}}{e B}$
• B
  $\sqrt{2 m(E-W_0) e B}$
• C
  $\frac{\sqrt{2 e(E-W_0)}}{m B}$
• D
  $\frac{\sqrt{2 m(E-W_0)}}{e B}$