In the following pair, determine whether the two represent resonance contributors of a single species or depict different substances. If two structures are not resonance contributors, explain why? a. N¨−N≡N: and N¨=N=N:N¨=N=N: 
b. N¨−N≡N:N¨−N≡N: and N¨−N=N¨N¨−N=N ¨ 
c. N¨−N≡N:N¨−N≡N: and N¨−N¨−N¨N¨−N¨ −N¨