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

Question

( A ) $(K-1)CV^2$
( B ) zero
( C ) $CV^2(K-1)/K$
( D ) $\frac{1}{2} (K-1) CV^2$