In Millikan's oil drop experiment, what is viscous force acting on an uncharged drop of radius $2.0 \times 10^{-5}\mathrm{~m}$ and density $1.2 \times 10^{3}\mathrm{~kg~m}^{-3}$? Take viscosity of liquid $= 1.8 \times 10^{-5}\mathrm{~N~s~m}^{-2}$. (Neglect buoyancy due to air).\\
(1) $5.8 \times 10^{-10}\mathrm{~N}$\\
(2) $3.9 \times 10^{-10}\mathrm{~N}$\\
(3) $1.8 \times 10^{-10}\mathrm{~N}$\\
(4) $3.8 \times 10^{-11}\mathrm{~N}$