A sphere of mass m strikes a vertical wall heading to the right and bounces off it now going left. The magnitude of the momentum of the sphere just before impact is pB and just after impact is pA. The sphere is in contact with the wall for time t. The magnitude of the average force exerted by the wall on the sphere is A. (pB – pA)/t.B. (pB + pA)/t.C. (pB – pA)/mt.D. (pB + pA)/mt.