A force F at an angle θ above the horizontal is used to pull a heavy suitcase of weight mg a distance d along a level floor at constant velocity. The coefficient of friction between the floor and the suitcase is μ. The work done by the frictional force is ________.(A) -Fd cos θ (B) - µ Fd cos θ (C) -µmgd (D) -µ mgd cos θ