The bob of simple pendulum is hanging vertically down from a fixed identical bob by means of a string of length l. If both bobs are charged with a charge q each , time period of the pendulum is : (ignore the radii of the bobs)

Question

$$\begin {array} {1 1} (a)\;2 \pi \sqrt {\frac{1}{g+ \bigg(\frac{q^2}{l^2m}\bigg)}} & \quad (b)\;2 \pi \sqrt {\frac{1}{g- \bigg(\frac{q^2}{l^2 m}\bigg)}} \\ (c)\;2 \pi \sqrt {\frac{l}{g}} & \quad (d)\;2 \pi \sqrt {\frac{1}{g- \bigg(\frac{q^2}{l}\bigg)}} \end {array}$$