For which equilibrium can an expression for a base dissociation constant, ${K_{\text{b}}}$, for the forward reaction be written?

A.     ${\text{N}}{{\text{H}}_3} + {{\text{H}}_3}{{\text{O}}^ + } \rightleftharpoons {\text{NH}}_4^ +  + {{\text{H}}_2}{\text{O}}$

B.     ${{\text{F}}^ - } + {{\text{H}}_2}{\text{O}} \rightleftharpoons {\text{HF}} + {\text{O}}{{\text{H}}^ - }$

C.     ${\text{NH}}_4^ +  + {\text{O}}{{\text{H}}^ - } \rightleftharpoons {\text{N}}{{\text{H}}_3} + {{\text{H}}_2}{\text{O}}$

D.     ${\text{HF}} + {\text{O}}{{\text{H}}^ - } \rightleftharpoons {{\text{H}}_2}{\text{O}} + {{\text{F}}^ - }$

B