What is the equilibrium constant expression for the reaction below?

${\text{2N}}{{\text{O}}_2}{\text{(g)}} \rightleftharpoons {{\text{N}}_2}{{\text{O}}_4}{\text{(g)}}$

A.     ${K_{\text{c}}} = \frac{{{{[{\text{N}}{{\text{O}}_2}]}^2}}}{{[{{\text{N}}_2}{{\text{O}}_4}]}}$

B.     ${K_{\text{c}}} = \frac{{[{{\text{N}}_2}{{\text{O}}_4}]}}{{[{\text{N}}{{\text{O}}_2}]}}$

C.     ${K_{\text{c}}} = \frac{{[{{\text{N}}_2}{{\text{O}}_4}]}}{{2[{\text{N}}{{\text{O}}_2}]}}$

D.     ${K_{\text{c}}} = \frac{{[{{\text{N}}_2}{{\text{O}}_4}]}}{{{{[{\text{N}}{{\text{O}}_2}]}^2}}}$

D