Consider the equilibrium below.

$${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COOH(aq)}} + {{\text{H}}_{\text{2}}}{\text{O(l)}} \rightleftharpoons {\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{CO}}{{\text{O}}^ - }{\text{(aq)}} + {{\text{H}}_{\text{3}}}{{\text{O}}^ + }{\text{(aq)}}$$

Which species represent a conjugate acid-base pair?

A.     ${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COOH}}$ and ${{\text{H}}_{\text{2}}}{\text{O}}$

B.     ${{\text{H}}_{\text{2}}}{\text{O}}$ and ${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{CO}}{{\text{O}}^ - }$

C.     ${{\text{H}}_{\text{3}}}{{\text{O}}^ + }$ and ${{\text{H}}_{\text{2}}}{\text{O}}$

D.     ${\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{CO}}{{\text{O}}^ - }$ and ${{\text{H}}_{\text{3}}}{{\text{O}}^ + }$

C