A block attached to a vertical spring oscillates in simple harmonic motion with period \(T_0\) and amplitude \(A\). The potential energy \(U\) of the block-spring-Earth system is defined to be zero at the equilibrium position (\(y = 0\)). The block is released from rest at displacement \(y = +A\) at time \(t = 0\). Which of the following best describes the graph of the potential energy \(U\) as a function of time \(t\) over the interval from \(t = 0\) to \(t = T_0\)?