Architectural Designers is creating a schematic for a bridge over the Columbia River whose arch is in the shape of a parabola. The road, 40 feet above the river surface, represents the directrix of the parabolic arch. The focus of the parabolic arch is located 20 feet above the river surface. In the schematic, the river surface will be positioned along the x-axis and the axis of symmetry for the downward opening parabolic arch will be positioned along the y-axis. Which equation best models the parabolic arch in the schematic?