From the question, we were made to understand that the collision between the two billiard balls was an elastic collision. Hence, an elastic collision is one in which the kinetic energy is conserved. Meaning the kinetic energy before the collision is still retained after the collision.
Kinetic energy before collision = kinetic energy after collision
Explanation: It can't be gas because gases are all around us and we can still see light. For solids it can't even move through a solid. Ex: If you're in a solid dome made of rock you can't get any light through. Let me know if I'm right!
The carpet has a buildup of electrons in it that get released onto your body when you rub across the floor while the hardwood doesn't have electrons built up inside of it.
