A uniform disk with mass \(M\) and radius \(R\) is mounted on a horizontal, frictionless axle. A light string is wrapped around the disk and attached to a block of mass \(m\). The block is released from rest and accelerates downward, causing the disk to rotate. The rotational inertia of the disk is \(I = \dfrac{1}{2}MR^2\). Which of the following is a correct expression for the angular acceleration \(\alpha\) of the disk as the block falls?