Radial oxygen abundance gradient. The blue dotted line refers to the model M 31B of Marcon-Uchida et al. (2010), the black dashed line to the M 31N model with a constant SF efficiency, fixed at the value of 2 Gyr^-1, and without threshold, the red solid line refers to the best fit model M 31R with a radial inflow of gas. The filled circles and relative error bars are the observed values from HII regions.