Which combination of the following statements is correct ?P: A gas cools upon expansion only when its Joule-Thomson coefficient is positive in the temperature range of expansion.Q: For a system undergoing a process, its entropy remains constant only when the process is reversible.R: The work done by a closed system in an adiabatic process is a point function.S: A liquid expands upon freezing when the slope of its fusion curve on Pressure-Temperature diagram is negative.

• A. P, R and S
• B. P and Q
• C. Q, R and S
• D. P, Q, and R