The upper half of an inclined plane with inclination $\varphi$ 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:

1. A
   $2\sin \varphi$
2. B
   $2\cos \varphi$
3. C
   $2\tan \varphi$
4. D
   $\tan \varphi$