Top panel: continuum-subtracted stacked spectrum of C III] in the sample (blue line) and the corresponding uncertainty (blue shaded region). The dashed red and green lines represent the best-Gaussian fit of O III] and He II, respectively, while the dashed black line represents the total fit of the blend lines. The vertical gray line indicates the position of C IV. Bottom panel: the O III] / He II line ratio as a function of C III] equivalent width. The blue square represents the O III] / He II ratio measured in the stack, yielding a value of 0.87±0.38. This value is compared to measurements from Amorín et al. (2017), Llerena et al. (2022) and Hu et al. (2024).