In part (a) of the figure, an air track cart attached to a spring rests on the track at the position x_equilibrium and the spring is relaxed. In (b), the cart is pulled to the position x_start and released. It then oscillates about x_equilibrium. Which graph correctly represents the potential energy of the spring as a function of the position of the cart?
A. 6   B. 5   C. 4   D. 3   E. 2   F. 1