A block of mass \(m\) slides on a horizontal frictionless surface with initial kinetic energy \(K_0\). The block undergoes a perfectly inelastic collision with a second block of mass \(3m\) that is initially at rest. The two blocks stick together and subsequently compress an ideal horizontal spring of spring constant \(k\) that is attached to a wall. Which of the following correctly compares the maximum elastic potential energy \(U_s\) stored in the spring to the initial kinetic energy \(K_0\) of the first block, and provides a correct justification?