A circular parallel–plate capacitor is connected to a battery in a circuit. The capacitor is fully charged before the battery is disconnected from the circuit. A uniform material of dielectric constant κ is inserted between the plates of the capacitor, effectively filling the space between the plates. Let Uold be the energy stored by the capacitor before the dielectric was inserted, while Unew is the energy stored after the dielectric was inserted. The ratio of Unew/Uold is
(A) 1/k2
(B) 1/k
(C) 1
(D) k
(E) k2
