An inclined plane makes an angle $30 ^{\circ}$ with horizontal. A groove $OA=5 m$ cut on the plane makes an angle $30 ^{\circ}$ with $OX$. A short smooth cylinder is free to slide down the groove under the influence of gravity. The time taken by the cylinder to reach from A to O is $(g=10 m/s^2)$

Question

 

$$(a)\;4\;s \quad (b)\;2\;s \quad (c)\;2 \sqrt 2 \;s \quad(d)\;1\;s$$