Show how resonance can occur in the following organic ions:

(a) Acetate ion, CH₃CO₂;

(b) Enolate ion, CH₂COCH₃^–, which has one resonance structure with a C=C double bond and an —O^– group on the central carbon atom;

(c) Allyl cation, CH₂ CHCH₂+;

(d) Amidate ion, CH₃ CONH^– (the O and the N atoms are both bonded to the second C atom).