Classify the following as acid-base reactions or oxidation-reduction reactions:
(a) Na₂S(aq) + 2HCl(aq) ⟶ 2NaCl(aq) + H₂S(g)
(b) 2Na(s) + 2HCl(aq) ⟶ 2NaCl(aq) + H₂(g)
(c) Mg(s) + Cl₂(g) ⟶ MgCl₂(s)
(d) MgO(s) + 2HCl(aq) ⟶ MgCl₂(aq) + H₂O(l)
(e) K₃P(s) + 2O₂(g) ⟶ K₃PO₄(s)
(f) 3KOH(aq) + H₃PO₄(aq) ⟶ K₃PO₄(aq) + 3H₂O(l)