What spectroscopic technique can be used to determine the molecular formula of a compound?

In addition to a carbonyl stretch, which of the following molecules exhibits two characteristic stretches at 2700 and 2800 cm^-1?

A) (CH₃CH₂) ₂CO
B) CH₃CH₂CH₂CHO
C) CH₃CH₂CO₂CH₃
D) CH₃CH₂CH₂CO₂H
E) CH₃CH₂CH₂COCl