Shells Losing Energy !

Electricity and Magnetism Level 4

An arrangement of two concentric conducting spherical shells charged initially such that the inner shell (radius = $a$ ) carries a net charge of $+Q$ while outer shell (radius $b$ ) carries net charge $0$ . If the switch " $S$ " is closed, how much electrostatic potential energy would the system lose?

\frac{Q^2}{4 \pi \epsilon_0 a}

\frac{Q^2}{4 \pi \epsilon_0 b}

\frac{Q^2}{8 \pi \epsilon_0 a}

\frac{Q^2 (b-a)}{4 \pi \epsilon_0 ab}

None of These

\frac{Q^2}{8 \pi \epsilon_0 b}