Mass-weighted 2D histogram of the fraction of C⁺ ions in the ²P_3/2 level (n_u/(n_u + n_l)) as a function of the dust temperature T_d. The solid red line represents the mass-weighted average of n_u/(n_u + n_l) for a given T_d bin, and the dashed line represents the median. The thin, dotted black line represents n_u/(n_u + n_l) = 2/3, that is, the maximum possible value. We show the result for MC2-HD at t_evol = 4 Myr, with 18 K ≤ T_d ≤ 55 K. On average, C+ reaches rough saturation at T_d ≃ 40 K. This confirms that, in vicinity of stars, the [CII] line is saturated, causing, in part, the [CII] deficit.