Which one of the following constitutes a group of the isoelectronic species?

• A
  $C_2^{2-}, O_2^-, CO, NO$
• B
  $NO^{+}, C_2^{2-}, CN^{-}, N_2$
• C
  $CN^{-}, N_2, O_2^{2-}, C_2^{2-}$
• D
  $N_2, O_2^-, NO^{+}, CO$