For a sample of perfect gas when its pressure is changed isothermally from pi to pf, the entropy change is given by
(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) $\Delta S=RT \ln({p_i\over p_f})$