A parallel plate condenser with a dielectric of dielectric constant K between plates has a capacity 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 this process is

• A

  $(\mathrm{K}-1){\mathrm{CV}}^2$

• B

  zero

• C

  $1/2(\mathrm{K}-1){\mathrm{CV}}^2$

• D

  ${\mathrm{CV}}^2(\mathrm{K}-1)/\mathrm{K}$