A hydrogen electrode X was placed in a buffer solution of sodium acetate ad acetic acid in the ratio `a:b` and another hydrogen electrode `Y` was placed in a buffer solution of sodium acetate ad acetic in the ratio `b:a` if reduction potential values for two cells are found to be `E_(1)` and `E_(2)` respectively w.r.t. standard hydrogen electrode, the `pK_(a)` value of the acid can be given as
(A) `(E_(1)-E_(2))/(0.118)`
(B). `-(E_(1)+E_(2))/(0.118)`
(C). `(E_(1))/(E_(2))xx0.118`
(D). `(E_(2)-E_(1))/(0.118)`