A(n) system is one in which energy moves freely in and out, but no matter enters or leaves the system.

A closed system is one in which energy moves freely in and out, but no matter enters or leaves the system.

Explanation:

In physics, a closed system refers to those that don't aloud certain specific transfer. In this case, a closed system only alows transfer of energy, but transfer of mass is not.

In other words, all systems that don't exchange mass, just energy, they are called closed systems.

It's important to notice that a closed system is not the same as an isolated system, because the last one doesn't a low any kind of transfer, not even mass. That's the difference.

Therefore, in this case, the word that completes the given statement is closed.

b.

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