6. A mass m = 4.0 kg is connected, as shown, by a light cord to a mass M = 6.0 kg, which slides on a smooth, horizontal surface. The pulley rotates about a frictionless axle and has a radius R = 0.12 m and a moment of inertia I = 0.090 kg·m². The cord does not slip on the pulley. What is the magnitude of the acceleration of m?
(A) 2.4 m/s² (B) 3.8 m/s² (C) 5.2 m/s² (D) 4.5 m/s² (E) 1.1 m/s²