$A$ pair of parallel metal plates are kept with a separation $d$. One plate is at a potential $+V$ and the other is at ground potential. $A$ narrow beam of electrons enters the space between the plates with a velocity $v_{0}$ and in a direction parallel to the plates. What will be the angle of the beam with the plates after it travels an axial distance $L$?

• A
  $\tan ^{-1}\left(\frac{eVL}{mdv_{0}}\right)$
• B
  $\tan ^{-1}\left(\frac{eVL}{mdv_{0}^{2}}\right)$
• C
  $\sin ^{-1}\left(\frac{eVL}{mdv_{0}}\right)$
• D
  $\cos ^{-1}\left(\frac{eVL}{mdv_{0}^{2}}\right)$