Cosmic time evolution of metallicity radial profiles. The solid lines in different colours represent Model B at 1 Gyr (cyan), 3 Gyr (red), 8.5 Gyr (blue), and 13.5 Gyr (present-day, black). The corresponding results predicted by Model A are shown as grey dashed lines. The red solid circles are the binned metallicity for HII regions from Bresolin et al. (2009), BSGs from Kudritzki et al. (2008), and RSGs from Gazak et al. (2015), while the blue solid diamonds denote the binned metallicity of PNe from Stasińska et al. (2013).