Block of mass m_0 is at rest at the top of a ramp, and the block's center of mass is at a height H_1 above the ground. Block X is then released from rest, and it slides down the ramp and collides with block Y of mass M_0, which is initially at rest and has its center of mass at a height h above the ground. At the moment before the collision, block X has a speed of V_0. After the collision, block Y traveles such that it lands at point P.

Consider the block X block Y system from the moment in time that block X is released from rest to the moment immediately after block X collides with block Y. Which of the following claims best describes the system?

The answer is A, because the total momentum of the system does not remain constant, the system is open. Can you explain why this is?