Identify the structure that is consistent with the following data set: molecular formula: C9H10O2, IR absorption at 1669 cm-1?
1) Ethanol (CH₃CH₂OH)
2) Acetone (CH₃COCH₃)
3) Benzoic acid (C₆H₅COOH)
4) Butanoic acid (CH₃CH₂CH₂COOH)