Which of the following statements is TRUE?

a. Momentum is always conserved within a system no matter how we choose the system.
b. In this lab, we define the system as the two carts, the track, and the earth.
c. In everyday life, elastic collisions are relatively easy to observe compared to completely inelastic collisions.
d. In everyday life, completely inelastic collisions are relatively easy to observe compared to elastic collision

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