A parallel-plate capacitor with plate separation d is connected to an ideal battery that maintains a constant potential difference V_0 across the plates. The bottom plate at y = 0 is connected to the negative terminal (at electric potential V = 0), and the top plate at y = d is connected to the positive terminal (at electric potential V = V_0). A solid slab of dielectric material with dielectric constant \kappa = 2 and thickness d/2 rests on the bottom plate, occupying the region 0 \le y \le d/2, while the upper region d/2 < y \le d is vacuum (\kappa = 1).

Which of the following graphs best represents the electric potential V as a function of vertical position y between the plates?