An ideal parallel-plate capacitor with plate separation \(d\) is connected to a battery that maintains a constant potential difference \(V_0\). A slab of linear dielectric material with dielectric constant \(\kappa > 1\) is slowly inserted between the plates until it completely fills the space. Two students disagree on the outcome: one asserts that dielectric polarization must reduce the net electric field between the plates, while the other asserts that the net electric field cannot change because the potential difference and plate separation remain fixed. Which of the following explanations correctly resolves this apparent contradiction?