A block of mass m is attached to a pulley disc of equal mass m, radius r by means of a slack string as shown. The pulley is hinged about its centre on a horizontal table and the block is projected with an initial velocity of 5 m/s. Its velocity when the string become staut will be
(A) 3 m/s
(B) 2.5 m/s
(C) 5/3 m/s
(D) 10/3 m/s