(a) Metal oxides that react with both acids and bases to produce salt and water are known as amphoteric oxides. For example, zinc reacts with an acid or a base to produce salt and water. Amongst the given oxides, zinc oxide (ZnO) and aluminium oxide (Al2O3) are amphoteric oxides. (b) Non-metals are electron acceptors. Hence, they cannot donate electrons to H+ ions to form hydrogen gas (H2).