A parallel plate capacitor with dielectric constant K between the plates has a capacitance C and is charged to a potential V volt. The dielectric slab is slowly removed from between the plates and then reinserted. The net work done by the system in the process is
a) zero
b) 1/2 (K-1) CV^2
c) CV^2 (K-1)/K
d)(K-1) CV^2
answer is option a. Please explain.