A thin string is wrapped several times around a cylinder kept on a rough horizontal surface. A boy standing at a distance l from the cylinder draws the string towards him as shown. The cylinder rolls without slipping . The length of the string passed through the hand of the boy while the cylinder reaches his hand is

Question

$$\begin {array} {1 1} (a)\;l & \quad (b)\;2l \\ (c)\;3l & \quad  (d)\;4l \end {array}$$