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

• A. \[2\,\tan \,\phi \]
• B. \[\,\tan \,\phi \]
• C. \[2\,sin\,\phi \]  
• D. \[2\,\cos \,\phi \]