In an ionic compound $A^{+} X^{-}$,the degree of covalent bonding is greatest when

• A
  $A^{+}$ and $X^{-}$ ions are small
• B
  $A^{+}$ is small and $X^{-}$ is large
• C
  $A^{+}$ and $X^{-}$ ions are approximately of the same size
• D
  $X^{-}$ is small and $A^{+}$ is large