A parallel-plate capacitor is charged and then disconnected from a power supply so that it carries a constant charge \(Q_0\). A dielectric slab with dielectric constant \(\kappa > 1\) that initially fills the entire region between the plates is pulled outward at a constant velocity by an external force until it is completely removed. Four students attempt to explain the resulting change in the electric potential energy stored in the capacitor and the physical mechanism responsible for that change. Which of the following explanations is physically correct?