Consider two ideal diatomic gases A and B at some temperature T. Molecules of the gas A are rigid, and have a mass m. Molecules of the gas B have an additional vibrational mode, and have a mass 4m. The ratio of the specific heats (CvA and CvB) of gas A and B, respectively is:
a. 7 : 9
b. 5 : 9
c. 3 : 5
d. 5 : 7