Which statement about systems is correct?

• A. A group of interacting parts that work together; energy can enter or leave a system.
• B. A system cannot exchange energy with its surroundings.
• C. A system is only defined by its energy content.
• D. A system must be isolated from its environment.

Answer: (A)

The idea being tested is that a system is a defined group of interacting parts that can exchange energy with its surroundings. This captures the essential boundary idea: the system is the set of components you’re studying, and energy can cross the boundary through heat or work, allowing the system to change state. Saying energy can enter or leave distinguishes it from an isolated setup where no exchange occurs; many real systems are open or closed, not isolated. Energy content alone does not define a system, since the system is defined by its boundary and the interactions of its parts, not just how much energy it holds. If a description says the system cannot exchange energy, or that it must be defined only by energy content, it misses the practical reality of how systems interact with their surroundings.