Left panels: average CO (4–3), CO (6–5) and CO (7–6) spectra convolved to 13.′′ 4 beam of each evolutionary class scaled to the median distance of the whole sample (d = 3.80 kpc). Right panels: same plot, but the average CO spectra are normalised by their peak intensity. The baseline level is indicated by the solid grey line and the black dashed line is placed at 0 km s⁻¹. The FWZP of the profiles are shown in the upper right side of the panels (in km s ⁻¹ units), together with the integrated intensities (S_int, in K kms⁻¹ units) of theCO profiles shown in the left panels.