Methylamine acts as a weak base when it reacts with water. For a diluted aqueous solution, what is the ${K_{\text{b}}}$ expression for this reaction?

A.     ${K_{\text{b}}} = \frac{{{\text{[C}}{{\text{H}}_{\text{3}}}{\text{NH}}_{\text{3}}^ + {\text{][O}}{{\text{H}}^ - }{\text{]}}}}{{{\text{[C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}{\text{]}}}}$

B.     ${K_{\text{b}}} = \frac{{{\text{[C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}{\text{][}}{{\text{H}}_{\text{2}}}{\text{O]}}}}{{{\text{[C}}{{\text{H}}_{\text{3}}}{\text{NH}}_{\text{3}}^ + {\text{][O}}{{\text{H}}^ - }{\text{]}}}}$

C.     ${K_{\text{b}}} = \frac{{{\text{[C}}{{\text{H}}_{\text{3}}}{\text{NH}}_{\text{3}}^ + {\text{][O}}{{\text{H}}^ - }{\text{]}}}}{{{\text{[C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}{\text{][}}{{\text{H}}_{\text{2}}}{\text{O]}}}}$

D.     ${K_{\text{b}}} = \frac{{{\text{[C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}{\text{]}}}}{{{\text{[C}}{{\text{H}}_{\text{3}}}{\text{NH}}_{\text{3}}^ + {\text{][O}}{{\text{H}}^ - }{\text{]}}}}$

A

Quite a few candidates included water in the expression, giving answer C.