A tiny spherical oil drop of radius r carrying a net charge q is balanced in still air with a vertical uniform electric field of strength E. When the field is switched off, the drop is observed to fall with terminal velocity  v , viscosity of the air $=\mathrm{\eta }$ and the density of oil = d. The magnitude of q is

• A

  $\mathrm{q}=\frac{6\mathrm{\eta rv}}{\mathrm{\pi E}}$

• B

  $\mathrm{q}=\frac{\mathrm{\pi \eta rv}}{6\mathrm{E}}$

• C

  $\mathrm{q}=\frac{6\mathrm{\pi \eta r}}{\mathrm{Ev}}$

• D

  $\mathrm{q}=\frac{6\mathrm{\pi \eta rv}}{\mathrm{E}}$