In a reversible reaction $A \overset{k_1}{\underset{k_2}{\longleftrightarrow}} B$,the initial concentrations of $A$ and $B$ are $a$ and $b$,and the equilibrium concentrations are $(a - x)$ and $(b + x)$ respectively. Express $x$ in terms of $k_1, k_2, a$,and $b$.

• A
  $\frac{k_1 a - k_2 b}{k_1 + k_2}$
• B
  $\frac{k_1 a - k_2 b}{k_1 - k_2}$
• C
  $\frac{k_1 a - k_2 b}{k_1 k_2}$
• D
  $\frac{k_1 a + k_2 b}{k_1 + k_2}$