Which represents a redox reaction?

A.     \({\text{NaH(s)}} + {{\text{H}}_{\text{2}}}{\text{O(l)}} \to {\text{NaOH(aq)}} + {{\text{H}}_{\text{2}}}{\text{(g)}}\)

B.     \({\text{CaC}}{{\text{O}}_{\text{3}}}{\text{(s)}} \to {\text{CaO(s)}} + {\text{C}}{{\text{O}}_{\text{2}}}{\text{(g)}}\)

C.     \({\text{CuC}}{{\text{l}}_{\text{2}}}{\text{(aq)}} + {{\text{K}}_{\text{2}}}{\text{S(aq)}} \to {\text{CuS(s)}} + {\text{2KCl(aq)}}\)

D.     \({\text{HCl(aq)}} + {\text{N}}{{\text{H}}_{\text{3}}}{\text{(aq)}} \to {\text{NH}}_{\text{4}}^ + {\text{C}}{{\text{L}}^ - }{\text{(aq)}}\)

A