In Milikan's oil drop experiment , an oil drop of radius r and charge q is held in equilibrium between the plates of a charged parallel-plate capacitor when the potential difference is V. To keep a drop of radius 2r and with a charge 2q in equilibrium between the plates the potential difference V required is

Question

$\begin{array}{1 1} V \\ 2V \\ 4V \\ 8V \end{array} $