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

Question

( A ) $ 2 \tan \phi$
( B ) $\tan \phi$
( C ) $2 \cos \phi$
( D ) $2 \sin \phi$