Answer:
a. Momentum is always conserved within a system no matter how we choose the system.
Explanation:
Within a system , the forces possible to be present are only internal. Presence of external force is ruled out. So we can easily apply law of conservation of momentum .
In a lab , we can not define a system which includes earth also because it is present outside lab also.
Elastic collisions are rare in every life because , in most collision , there is loss of energy
