A parallel-plate capacitor with distance of separation between the plates, d, is fully charged and the terminals are disconnected from the battery. the plates are then pulled apart to increase their separation to 2d. finally, a dielectric with?= 4 is inserted between the plates. which of the following statements is true
a. The capacitance of the capacitor decreases to half its original value.
b. The energy stored in the capacitor increases by a factor of 4.
c. The potential difference across the plates remains unchanged.
d. The electric field between the plates increases by a factor of 4.