A wheel is rotating at $900 \;rpm$ about its axis. When the power is cut off, it comes to rest in 1 minute. The angular retardation in $rad/s^2$ is

Question

\[\begin {array} {1 1} (a)\;\frac{\pi}{2} & \quad (b)\;\frac{\pi}{4} \\ (c)\;\frac{\pi}{6} & \quad  (d)\;\frac{\pi}{8} \end {array}\]