Concept:A closed and isolated system is one in which the law of conservation of momentum is valid.Explanation:For a system to be closed and isolated, two conditions must hold.There must be no net external force acting on the system.The total mass of the system must remain constant, meaning no mass enters or leaves the system.When these two conditions are satisfied, the total momentum of the system remains unchanged.Answer:A. No external force acts on the system and there is no change in mass of the system.