Which of the following pairs consists of two resonance contributors of the same species?

A $CH X_{2} = CH - O X^{-}$ and $X_{-} X_{2}^{2 -} CH X_{2} - CH = O$
B $CH X_{3} - CH X_{2} X^{+}$ and $CH X_{2} = CH X_{2}$
C $CH X_{3} - CO - CH X_{3}$ and $CH X_{3} - C (OH) = CH X_{2}$
D $CH X_{3} CH X_{2} OH$ and $CH X_{3} OCH X_{3}$

Answer: A

Resonance contributors differ only in electron positions. The enolate pair differs only in the placement of the lone pair and the pi bond, so it is a pair of contributors. Acetone and propen-2-ol are tautomers (a hydrogen moves), and ethanol and dimethyl ether are different compounds.

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Which of the following pairs consists of two resonance contributors of the same species?

Answer: A

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