Panel A: the evolution of the surface C abundance in the stars with the masses 0.8 M_⊙ (blue), 0.7 M_⊙ (green), and 0.65 M_⊙ (red) and initial chemical compositions from the second, third, and fourth columns of Table 1, respectively, calculated from the MS to the RGB tip. The RGB extra mixing is modelled using the thermohaline diffusion coefficient (equation 25) from Denissenkov (2010) with the salt-finger aspect ratio a = 7 that produces the maximum possible decrease of [C/Fe]. The results of these calculations are compared with the observational data for M13 MS turn-off and subgiant stars (the open blue diamonds) and RGB stars (the other symbols, except the red star symbols which represent AGB stars) from Briley et al. (2002). Panel B: the same as in panel A, but for the stars of the single stellar population GC NGC5466 observed by Shetrone et al. (2010) and a = 10. Panel D: the same as in panel B, but here we have also used the parametric prescription for the RGB extra mixing with the mixing depth log₁₀(r_mix/R_⊙) = −1.35 (same as for the thermohaline mixing) and diffusion coefficient D_mix = αK, where K is the radiative diffusivity and α = 0.02 (magenta line) and 0.03 (black line). Panel C: the same as in panel D, but for the M 13 stars with the dashed and solid curves representing the cases of α = 0.02 and 0.03.