A block is gently placed on a conveyor belt moving horizontally with constant speed. After 4 s the velocity of the block becomes equal to the velocity of belt. If the coefficient of friction between the block and the belt is 0.2, then velocity of the conveyor belt is
(a) $2 \mathrm{~ms}^{-1}$
(b) $4 \mathrm{~ms}^{-1}$
(c) $6 \mathrm{~ms}^{-1}$
(d) $8 \mathrm{~ms}^{-1}$