In a reversible reaction `Aunderset(K_2)overset(K_1)hArrB` the initial concentration of A and B are a and b in moles per litre and the equilibrium concentrations are (a-x) and (b+x) respectively, Express x in terms of `K_1, K_2, a and b`.
A. `(K_1a-K_2b)/(K_1+K_2)`
B. `(K_1a-K_2b)/(K_1-K_2)`
C. `(K_1a-K_2b)/(K_1K_2)`
D. `(K_1a+K_2b)/(K_1+K_2)`