In a dilute solution of elements X, Y etc. in liquid iron, the effect of Y on the activity co-efficient (fx) and the activity (hx) of X with respect to the Henrian 1wt% standard state is taken into account by the activity interaction co-efficient e^y_x, which is

A. $$e_x^y = \frac{{\partial \log hx}}{{\partial \left[ {\% Y} \right]}}$$

B. $$e_x^y = \frac{{\partial \log fx}}{{\partial \left[ {\% Y} \right]}}$$

C. $$e_x^y = \frac{{\partial hx}}{{\partial \left[ {\% Y} \right]}}$$

D. $$e_x^y = \frac{{\partial fx}}{{\partial \left[ {\% Y} \right]}}$$

Answer: Option B