[C/N] distributions normalised to the histogram area of the synthetic population simulated with the BGM with the effects of thermohaline instability (blue solid histograms) and without (grey shaded histograms) during the red giant branch. Giant stars are divided into three groups: low-RGB stars (before the RGB-bump luminosity, top panel), upper-RGB stars (after the RGB-bump luminosity, middle panel), and clump/early-AGB stars (bottom panel).