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

$$\frac{a^2 - x^2}{x^2} \times p$$