A block of mass \(M\) is placed on a horizontal, frictionless surface and pushed against an ideal spring with spring constant \(k\), compressing it a distance \(d\) from its equilibrium position. The block is released from rest and is launched by the spring. The block loses contact with the spring at the moment the spring reaches its equilibrium position. Which of the following is a correct expression for the average power delivered to the block by the spring during the time interval from release until the block loses contact?