For the reaction equilibrium $N_2O_4(g) \rightleftharpoons 2NO_2(g)$,the concentrations of $N_2O_4$ and $NO_2$ at equilibrium are $4.8 \times 10^{-2} \ mol \ L^{-1}$ and $1.2 \times 10^{-2} \ mol \ L^{-1}$ respectively. The value of $K_c$ for the reaction is:

• A
  $3.3 \times 10^2 \ mol \ L^{-1}$
• B
  $3 \times 10^{-1} \ mol \ L^{-1}$
• C
  $3 \times 10^{-3} \ mol \ L^{-1}$
• D
  $3 \times 10^3 \ mol \ L^{-1}$