If sand of mass M is allowed to drop from a stationary hopper into a conveyor belt at a rate given by dM/dt, the magnitude of the force required to keep the belt moving at a constant speed of v is
(a) v.v(dM/dt) (b) v.(dM/dt)
(c) (1/v).(dM/dt) (d) (1/v.v).(dM/dt)
