$A$ tiny spherical oil drop carrying a net charge $q$ is balanced in still air with a vertical uniform electric field of strength $\frac{81 \pi}{7} \times 10^5 \text{ Vm}^{-1}$. When the field is switched off,the drop is observed to fall with a terminal velocity $2 \times 10^{-3} \text{ ms}^{-1}$. Given $g = 9.8 \text{ ms}^{-2}$,viscosity of the air $\eta = 1.8 \times 10^{-5} \text{ Ns m}^{-2}$,and the density of oil $\rho = 900 \text{ kg m}^{-3}$,the magnitude of $q$ is:

• A
  $1.6 \times 10^{-19} \text{ C}$
• B
  $3.2 \times 10^{-19} \text{ C}$
• C
  $4.8 \times 10^{-19} \text{ C}$
• D
  $8.0 \times 10^{-19} \text{ C}$