= Solution
\b[A: conjugated <ester>; B: unconjugated <ester>; C: terminal <alkyne> <acetal>.]
A has a strong <ester> C=O band at roughly $1715$–$1730\,\mathrm{cm^{-1}}$, lowered by <conjugation> with C=C. B's unconjugated <ester> <carbonyl> is nearer $1735$–$1750\,\mathrm{cm^{-1}}$; it also has terminal-alkene features, including the approximately $990/910\,\mathrm{cm^{-1}}$ pair.
C has \b[no <carbonyl> band]. Its terminal <alkyne> gives a relatively sharp sp C-H stretch near $3300\,\mathrm{cm^{-1}}$ and a usually weaker C≡C feature near $2100$–$2150\,\mathrm{cm^{-1}}$. C-O stretching alone is less diagnostic because <esters> and the <acetal> all contain C-O bonds. The numbers are characteristic ranges, not exact frequencies inferred from an unprinted IR trace.
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