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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.