8. A ball of mass m is dropped from rest from a height h and collides elastically with the floor, rebounding to its original height. What is the magnitude of the average force applied by the floor on the ball during the time the ball is in contact with the floor? (A) Zero (B) mg (C) 2mg (D) 4mg (E) It cannot be determined without knowing the length of time that the ball is in contact with the floor.