Which combination of the following statements is correct?
P: A gas cools upon expansion only when its Joule-Thomson Co-efficient 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 liqauid expand upon freezing when the slope of its fusion curve on Pressure Temperature diagram is negative
(a) R and S (b) P and Q
(c) Q, R and S (d) P, Q and R