Task: Question Answering
Input (English): Background Paragraph: Another thing we can see in this animation is that particles are constantly colliding with one another. One assumption that we make when talking about collisions between gas particles is that they are completely elastic collisions. In an elastic collision , momentum is conserved, which means that none of the kinetic energy of the colliding particles is lost in some other form (such as the emission of light). This makes sense, because if energy were lost in collisions, the speeds of the particles would gradually decrease over time, and eventually everything would condense down into a solid form. 
Story: Dan is in his physics lab. He is experimenting with colliding particles. He has two samples of particles, sample A and sample B. Sample A contains gas particles, but sample B does not contain gas particles. 
Question: In which sample the momentum would be conserved, sample A or sample B?
Output (English): sample A.