The internal energy of the system decreases in an adiabatic process. Which of the following must be true regarding this process?

Explanation

According to the first law of thermodynamics, Q = ΔU + W, where Q is the heat added to the system, ΔU is the change in internal energy, and W is the work done by the system. In an adiabatic process, Q = 0, so the equation simplifies to ΔU = -W. This means that if the internal energy decreases, W must be positive, indicating that work is done by the system on the surroundings. Thus, the correct answer is that work is done by the system. The other options are incorrect because they either suggest heat transfer (which does not occur in an adiabatic process) or involve potential energy, which is not directly related to the described process.