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.