Which represents an empirical formula?

A.     ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}}$

B.     ${{\text{B}}_{\text{2}}}{{\text{H}}_{\text{6}}}$

C.     ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$

D.     ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{6}}}$

C