If we examine a ball in free fall, we find that the momentum of the ball is not constant. This is not a violation of the law of conservation of momentum because

A) the force of gravity acts on the ball.
B) the ball experiences an external force.
C) the system is not closed.
D) All of the above.
I think the answer is A because if the system is not closed or the ball experiences an external force it wouldn't be considered law of conservation of momentum. Am I right?

You are partially correct. The correct answer is D) All of the above.

The law of conservation of momentum states that the total momentum of a closed system remains constant if no external forces act on it. However, in the case of a ball in free fall, the force of gravity is acting on the ball, which is an external force.

Since the ball is not in a closed system because it is subject to the force of gravity, the momentum of the ball is not constant. Therefore, the law of conservation of momentum is not violated.