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