The reaction $A(g) \rightleftharpoons B(g) + C(g)$ was initiated with the amount $a$ of $A(g)$. At equilibrium, it is found that the amount of $A(g)$ remaining is $(a-x)$ at a total pressure of $p$. The equilibrium constant $K_{p}$ of the reaction can be calculated from the expression:

• A
  $\frac{x^{2}}{a^{2}+x^{2}} \times p$
• B
  $\frac{x^{2}}{a^{2}-x^{2}} \times p$
• C
  $\frac{a+x^{2}}{x^{2}} \times p$
• D
  $\frac{a^{2}-x^{2}}{x^{2}} \times p$