The graph below shows the velocity versus time for a bullet as it is fired from a gun, travels a short distance, and enters a block of wood. Compute the acceleration at the times marked a, b, and c. (Indicate the direction with the sign of your answer. Assume the bullet's initial motion is in the positive direction.)

v (m/s) 800 700 600 500 400 300 200 100 a b C t (s) 0.001 0.002 0.003