A block of mass \(m\) on a horizontal, frictionless surface is attached to the end of a non-ideal spring. The block is pulled to position \(x = -x_0\) and released from rest at time \(t = 0\). The graph shows the horizontal force \(F_x\) exerted by the spring on the block as a function of the block's position \(x\) between \(x = -x_0\) and \(x = +x_0\).

Which of the following statements correctly describes the motion of the block?