A block of mass m is on an inclined plane of angle $$\theta$$. The coefficient of friction between the block and the plane is $$\mu$$ and $$\tan \theta > \mu$$. The block is held stationary by applying a force P parallel to the plane. The direction of force pointing up the plane is taken to be positive. As P is varied from $$P_1 = mg(\sin \theta - \mu \cos \theta)$$ to $$P_2 = mg(\sin \theta + \mu \cos \theta)$$, the frictional force f versus P graph will look like