Which of the oxides is not a mixed oxide?

• A

  $\mathrm{Co}{\left({\mathrm{Al}}_2{\mathrm{O}}_2\right)}_2$

• B

  ${\mathrm{Mn}}_3{\mathrm{O}}_4$

• C

  ${\mathrm{Pb}}_3{\mathrm{O}}_4$

• D

  ${\mathrm{C}}_3{\mathrm{O}}_2$