1. Concept: A closed and isolated system is one where momentum is conserved.2. Explanation: For momentum to be conserved (i.e., for a system to be closed and isolated), two conditions must be met:
There should be no net external force acting on the system.
The mass of the system must remain constant (no mass entering or leaving).
3. Answer: A. No external force acts on the system and there is no change in mass of the system.