For the decomposition reaction $N_2O_4 \rightleftharpoons 2NO_2$ at constant temperature,the equilibrium constant is given by $K_p = \frac{4x^2P}{1 - x^2}$,where $P$ is the total pressure and $x$ is the degree of dissociation. Which of the following statements is correct?

• A
  $K_p$ increases with an increase in $P$.
• B
  $K_p$ increases with an increase in $x$.
• C
  $K_p$ decreases with a decrease in $x$.
• D
  $K_p$ remains constant with changes in $P$ and $x$.