$A$ monochromatic light is incident on a metallic plate having work function $\phi$. An electron,emitted normally to the plate from a point $A$ with maximum kinetic energy,enters a constant magnetic field,perpendicular to the initial velocity of electron. The electron passes through a curve and hits back the plate at a point $B$. The distance between $A$ and $B$ is (Given: The magnitude of charge of an electron is $e$ and mass is $m$,$h$ is Planck's constant,and $c$ is velocity of light. Take the magnetic field exists throughout the path of electron).

• A
  $\sqrt{2 m(\frac{hc}{\lambda}-\phi)} / eB$
• B
  $\sqrt{m(\frac{hc}{\lambda}-\phi)} / eB$
• C
  $\sqrt{8 m(\frac{hc}{\lambda}-\phi)} / eB$
• D
  $2 \sqrt{m(\frac{hc}{\lambda}-\phi)} / eB$