9. While the carbonyl IR stretching frequency for simple aldehydes, ketones, and carboxylic acids is about $1710~\mathrm{cm}^{-1}$, the carbonyl stretching frequency for amides is about ______.
(A) $1700~\mathrm{cm}^{-1}$
(B) $1735~\mathrm{cm}^{-1}$
(C) $1800~\mathrm{cm}^{-1}$
(D) $1660~\mathrm{cm}^{-1}$
(E) $2200~\mathrm{cm}^{-1}$

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