How many pairs below represent a conjugate acid/base pair? HCl / OCl^- H_2SO_…

How many pairs below represent a conjugate acid/base pair?
HCl / OClHCl\ /\ OCl^-
H2SO4/SO42H_2SO_4/SO_4^{2-}
NH4+/NH3NH_4^{+^{ }}/NH_3
H3O+/ OHH_3O^+/\ OH
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