For a sample of perfect gas when its pressure is changed isothermally from *p _{i}* to

*p*, the entropy change is given by

_{f}(a) $\Delta S=nR \ln({p_f\over p_i})$

(b) $\Delta S=nR \ln({p_i\over p_f})$

(c) $\Delta S=nRT \ln({p_f\over p_i})$

(d) $\Delta S=RT \ln({p_i\over p_f})$