A parallel plate condenser with a dielectric of dielectric constant $K$ between the plates has a capacity $C$ and is charged to a potential of $V$ volts. The dielectric slab is slowly removed from between the plates and then reinserted. The net work done by the system in this process is

Question

$\begin {array} {1 1} (A)\;\large\frac{1}{2} \normalsize (K – 1)CV^2 & \quad (B)\;CV^2 (K – 1)/K \\ (C)\;(K – 1) CV^2 & \quad (D)\;Zero \end {array}$