For a sample of perfect gas when its pressure is changed isothermally from $$p_i$$ to $$p_f$$, the entropy change is given by

A

$$\triangle s = nR \ln \left(\frac{p_i}{p_f}\right)$$

B

$$\triangle s = nRT \ln \left(\frac{p_f}{p_i}\right)$$

C

$$\triangle s = RT \ln \left(\frac{p_i}{p_f}\right)$$

D

$$\triangle s = nR \ln \left(\frac{p_f}{p_i}\right)$$