According to the scientific observation that energy is conserved in a "closed system,” no machine can do work on its surrounding environment forever. This observation has been made over and over again in all sciences, and no example has been found that disproves the fact of conservation of energy.

The statement describes a consistent, repeatedly observed pattern: energy is conserved in a closed system. In science, a law summarizes a pattern or relationship that has been supported by extensive evidence. A theory explains why or how patterns occur, while a hypothesis is a tentative, testable proposal. So the correct choice is “a law.”

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