Which pair would not be expected to form precipitate when solutions are mixed?

• A

  ${\mathrm{K}}^{\oplus },{\mathrm{SO}}_4^{2-}$

• B

  ${\mathrm{Na}}^{\oplus },{\mathrm{S}}^{2-}$

• C

  ${\mathrm{Ag}}^{\oplus },{\mathrm{NO}}_3^{\ominus }$

• D

  ${\mathrm{Al}}^{3+},{\mathrm{OH}}^{\ominus }$