At constant temperature,the equilibrium constant $(K_p)$ for the decomposition reaction $N_2O_4 \rightleftharpoons 2NO_2$ is expressed by $K_p = \frac{4x^2P}{1 - x^2}$,where $P = \text{pressure}$,$x = \text{extent of decomposition}$. Which one of the following statements is true?

• A
  $K_p$ increases with increase of $P$
• B
  $K_p$ increases with increase of $x$
• C
  $K_p$ increases with decrease of $x$
• D
  $K_p$ remains constant with change in $P$ and $x$