Which of the following pair of ions makes the water hard ?

• A

  ${\mathrm{Na}}^{+}, {\mathrm{SO}}_4^{2-}$

• B

  ${\mathrm{K}}^{+}, {\mathrm{HCO}}_3^{-}$

• C

  ${\mathrm{Ca}}^{2+}, {\mathrm{NO}}_3^{-}$

• D

  ${\mathrm{NH}}_4^{+}, {\mathrm{Cl}}^{-}$