A parallel-plate capacitor is filled with a dielectric material of dielectric constant \(\kappa = 3.0\). The capacitor is connected to a power supply until it acquires a potential difference \(V_0\) and stored electrostatic energy \(U_0\), and is then disconnected from the power supply, leaving it isolated. The dielectric slab is then completely removed from between the plates. Which of the following correctly pairs the new potential difference \(V_f\) across the plates and the new stored electrostatic energy \(U_f\) in terms of \(V_0\) and \(U_0\)?