What is the correct expression for the ionic product constant of water, ${K_{\text{w}}}$?

A.     ${K_{\text{W}}} = \frac{{{\text{[}}{{\text{H}}^ + }{\text{]}}}}{{{\text{[O}}{{\text{H}}^ - }{\text{]}}}}$

B.     ${K_{\text{W}}}{\text{ = }}\frac{{{\text{[}}{{\text{H}}_2}{\text{O]}}}}{{{\text{[}}{{\text{H}}^ + }{\text{][O}}{{\text{H}}^ - }{\text{]}}}}$

C.     ${K_{\text{W}}} = {\text{[}}{{\text{H}}^ + }{\text{]}} + {\text{[O}}{{\text{H}}^ - }{\text{]}}$

D.     ${K_{\text{W}}} = {\text{[}}{{\text{H}}^ + }{\text{][O}}{{\text{H}}^ - }{\text{]}}$

D