Which pairs of ions are responsible for hardness of water?

• A
  $Ca^{2+}$ and $Mg^{2+}$
• B
  $Na^{+}$ and $K^{+}$
• C
  $Cl^{-}$ and $SO_4^{2-}$
• D
  $Fe^{2+}$ and $Mn^{2+}$