From the two pairs (a) and (b) of resonance structures given below, designate the ones that would contribute most to the resonance hybrid.
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Resonance structures contribute more to the hybrid if they have lower energy, which is favored by having more bonds, complete octets, and minimal charge separation. In pair (a), structure II has a complete octet on all atoms and no charge separation, while I has a positive charge on carbon. In pair (b), structure IV has a complete octet on all atoms and no charge separation, while III has a positive charge on carbon. Therefore, II and IV are the most stable and contribute the most.
Final Answer: II & IV