Using two metal plates, a physics student builds a parallel-plate capacitor. The gap between the two plates is set at exactly 4.0 mm apart, and each holds an equal but opposite charge with a magnitude of 120 nC. The medium between them is a vacuum, while they experience an electric field having an intensity of 7.0 × 10⁶ V/m. Determine (a) the voltage difference across them, (b) the surface area per plate, and (c) the capacitor's capacitance.