A child is playing on a trampoline jumping up and down. She is very skillful in playing on the trampoline, and at every bounce she reaches the same height always. Which of the followings is correct? (A) The elastic potential energy of the trampoline is constant and never changes while she is playing.
(B) Her gravitational potential energy is constant and never changes while she is playing.
(C) Her gravitational potential energy changes constantly, and reaches maximum when she reaches the maximum height.
(D) The elastic potential energy of the trampoline changes constantly, and reaches maximum when she reaches the maximum height.