Which of the following statements is correct about $CO_3^{2-}$ ?
$A$. The hybridisation of the central atom is $sp^3$.
$B$. Its resonance structure has one $C-O$ single bond and two $C=O$ double bonds.
$C$. The average formal charge on each oxygen atom is $0.67$ units.
$D$. All $C-O$ bond lengths are equal.

• A
  $A$
• B
  $A$ and $C$
• C
  $B$ and $D$
• D
  $C$ and $D$