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A tiny spherical oil drop carrying a net charge \( \mathrm{q} \) is balanced in still air will a vertical uniform electric field of strength \( \frac{81 \pi}{7} \times 10^{5} \mathrm{Vm}^{-1} \). When the field is switched off, the drop is observed to fall with terminal velocity \( 2 \times 10^{-3} \mathrm{~ms}^{-1} \). (Given : \( \mathrm{g}=9.8 \mathrm{~ms}^{-2} \), viscosity of the air \( =1.8 \times 10^{-5} \mathrm{Nsm}^{-2} \) and the density of oil \( =900 \mathrm{kgm}^{-3} \) ) the magnitude of \( \mathrm{q} \) is
(a) \( 1.6 \times 10^{-19} \mathrm{C} \)
(b) \( 3.2 \times 10^{-19} \mathrm{C} \)
(c) \( 4.8 \times 10^{-19} \mathrm{C} \)
(d) \( 8.0 \times 10^{-19} \mathrm{C} \)
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