Question

a system that doesnt gain or lose mass is said to be _.
a redundant system
an external system
an open system
a closed system

Explanation:

A closed system is defined as a system that does not exchange mass with its surroundings. A redundant system refers to having backup components. An external system is not a standard term in this context. An open system exchanges both mass and energy. Since the question is about a system that doesn't gain or lose mass, the key is the definition of a closed system.

Answer:

D. A closed system