31. In a baseball game, the pitcher throw a ball (150 g) at a speed of 30.0 m/s. The batter hit it straight back with a speed of40.0 m/s. What is the average force exerted by the bat if the bat-ball contact time is 0.005 sec?
(A) 300 N
(B) 14000 N
(C) 1200 N
(D) 2100 N
(E) 900 N