A gun with a mass of 5 kg fires a bullet with a mass of 0.01 kg. If the bullet leaves the gun with a velocity of 500 m/s and the gun experiences a force in the backward direction, what is the velocity of the gun in the backward direction?
A. -1 m/s
B. 8 m/s
C. -0.6 m/s
D. -2 m/s