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. `mu=(1)/(tantheta)`
B. `mu=(2)/(tantheta)`
C. `mu=2tantheta`
D. `mu=tantheta`
