Two large circular discs separated by a distance of $0.01 \mathrm{m}$ are connected to a battery via a switch as shown in the figure. Charged oil drops of density $900 \mathrm{kg} {\mathrm{m}}^{-3}$ are released through a tiny hole at the centre of the top disc. Once some oil drops achieve terminal velocity, the switch is closed to apply a voltage of $200 \mathrm{V}$ across the discs. As a result, an oil drop of radius $8\times {10}^{-7} \mathrm{m}$ stops moving vertically and floats between the discs. The number of electrons present in this oil drop is __________. (neglect the buoyancy force, take acceleration due to gravity $=10 \mathrm{m} {\mathrm{s}}^{-2}$ and charge on an electron (e) $=1.6\times {10}^{-19} \mathrm{C}$)

Hence, the correct answer is 6.