The upper half of an inclined plane with inclination of $\phi$ is perfectly smooth while the lower half is rough. A body starting from rest at top will again come to rest at bottom if coefficient of friction for lower half is given by

Question

\[(a)\;2 \sin \phi \quad (b)\;2 \cos \phi \quad (c)\;\tan \phi \quad (d)\;2 \tan \phi\]