3m A block of mass 3m can move without friction on a horizontal table. This block is attached to another block of mass m by a cord that passes over a frictionless pulley, as shown above. If the masses of the cord and the pulle are negligible, what is the magnitude of the acceleration of the descending block? (A) g/4 (B) g/3 (C) 2g/3 (D) g