The specific heats, Cp and Cv of a gas of diatomic molecules, A, are given (in units of J mol-1 k-1) by 29 and 22 respectively. Another gas of diatomic molecules, B, has the corresponding values 30 and 21. If they are treated as ideal gases, then:

• A. A is rigid but B has a vibrational mode.
• B. A has a vibrational mode but B has none.
• C. A has one vibrational mode and B has two.
• D. Both A and B have a vibrational mode each.