A particle constrained to move along the x-axis is subject to a conservative force associated with the potential energy function U(x) shown in the graph. The potential energy increases linearly from U = 0 at x = 0 to U = U_0 at x = 2d, remains constant at U = U_0 from x = 2d to x = 3d, and decreases linearly from U = U_0 to U = 0 from x = 3d to x = 4d. Letting F_0 = \dfrac{U_0}{2d}, which of the following graphs best represents the component of the net force F_x exerted on the particle as a function of position x?