# A oil drop having a mass $4.8 \times 10^{10}\;g$ and charge $2.4 \times 10^{-18}\;C$ stands still between two charged horizontal plates separated by a distance of 1 cm. If now the polarity of the plates is changed, instantaneous acceleration of the drop is : $(g= 10 ms^{-2}$)

$\begin {array} {1 1} (a)\;5\;ms^{-2} & \quad (b)\;10\;ms^{-2} \\ (c)\;15\;ms^{-2} & \quad (d)\;20\;ms^{-2} \end {array}$

$(b)\;10 ms^{-2}$