Two students perform an experiment with soccer balls. They kick two soccer balls so that they collide. They measure the total momentum of the two soccer balls before and after the collision. Which statement explains why the total momentum of the soccer balls before and after the collision may not be the same?
A. The soccer balls exert forces on each other when they collide.
B. The system has external forces, such as friction and air resistance, acting on it.
C. The system does not have any external forces.
D. The soccer balls each have a different momentum after the collision than before the collision.