Effect of spectral resolution and rotational velocity on three sets of optical lines classically used to determine C, N, and O abundances. In each panel, the initial CMFGEN model has T_eff = 31 000 K, log g = 3.6 and 0.2 Z_⊙. The model is degraded to a resolution of either 6000 (typical for our X-shooter UVB spectra) or 80 000 and further convolved with three rotational velocities (20, 100, and 300 km s⁻¹). No additional macro-turbulent broadening is considered. Also plotted is the UV C III 1176 line. Here the spectral resolution is that of the STIS E140M grating (R ∼ 45 000).