Ammonia acts as a weak base when it reacts with water. What is the ${K_{\text{b}}}$ expression for this reaction?

A.     $\frac{{[{\text{NH}}_4^ + ][{\text{O}}{{\text{H}}^ - }]}}{{[{\text{N}}{{\text{H}}_3}][{{\text{H}}_2}{\text{O]}}}}$

B.     $\frac{{[{\text{N}}{{\text{H}}_3}{\text{][}}{{\text{H}}_2}{\text{O]}}}}{{{\text{[NH}}_4^ + {\text{][O}}{{\text{H}}^ - }{\text{]}}}}$

C.     $\frac{{[{\text{N}}{{\text{H}}_3}]}}{{[{\text{NH}}_4^ + ][{\text{O}}{{\text{H}}^ - }]}}$

D.     $\frac{{[{\text{NH}}_4^ + ][{\text{O}}{{\text{H}}^ - }]}}{{[{\text{N}}{{\text{H}}_3}]}}$

D