An ideal gas of mass m and temperature T1 undergoes a reversible isothermal process from an initial pressure P1 to final pressure P2. The heat loss during the process is Q. The entropy change DS of the gas is
(a) mR In$\left ( \frac{p_{2}}{p^{_{1}}} \right )$ 
(b) $mR In\left ( \frac{p_{1}}{p^{_{2}}} \right )$
(c) $mR In\left ( \frac{p_{2}}{p^{_{1}}} \right ) -\frac{Q}{p_{1}}$ 
(d) zero