A ball is dropped on the floor from a height of 10 m. It rebounds to a height of 2.5 m. If the ball is in contact with the floor for 0.01 sec, the average acceleration during contact is
A. 2100 m/sec² downward
B. 2100 m/sec² upward
C. 1400 m/sec²
D. 700 m/sec²