$A$ simple pendulum has a bob with mass $m$ and charge $q$. The pendulum string has negligible mass. When a uniform and horizontal electric field $\vec{E}$ is applied, the final tension in the string changes. The final tension in the string, when the pendulum attains an equilibrium position, is . . . . . . . ($g$: acceleration due to gravity)

• A
  $mg-qE$
• B
  $mg+qE$
• C
  $\sqrt{m^{2}g^{2}+q^{2}E^{2}}$
• D
  $\sqrt{m^{2}g^{2}-q^{2}E^{2}}$