A block of mass \(M\) slides along a horizontal frictionless surface at a constant speed \(v_0\). The block is moving toward an ideal spring with spring constant \(k\) that is attached to a rigid wall. Two cases are considered:

Case 1: The block of mass \(M\) hits the spring and compresses it to a maximum potential energy \(U_1\).

Case 2: The block of mass \(M\) first collides with and sticks to a second stationary block of mass \(M\). The two-block system then hits the spring and compresses it to a maximum potential energy \(U_2\).

Which of the following correctly compares \(U_2\) to \(U_1\) and provides a correct justification?