A Parallel-Plate Capacitor Has a Capacitance of 10 MF and Charged

A parallel-plate capacitor has a capacitance of 10 mF and charged with a 20 V power supply. The power supply is then removed and a dielectric of dielectric constant 4 is used to fill the space between the plates. The energy now stored by the capacitor is

A) 250 mJ.
B) 125 mJ.
C) 500 mJ.
D) zero.
E) 62.5 mJ.