9. A block of mass 3 m 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 pulley are negligible, what is the magnitude of the acceleration of the descending block?
(A) g/4
(B) g/3
(C) 2g/3
(D) g