Review the equilibrium and choose the correct statement
HClO4 + H2O ⇌ H3O+ + ClO−4
HClO4 is the conjugate acid of H2O
H3O+ is the conjugate base of H2O
H2O is the conjugate acid of H3O+
ClO−4 is the conjugate base of HClO4
Here HClO4 and ClO−4 constitute conjugate acid and base pair.