Left panel: [C/O]–[O/H] correlation based on Garnett et al. (1995) (dot-dashed black), Nicholls et al. (2017) (dashed black), and Amarsi et al. (2019) observations, the latter using the best-fit relations of Sharda et al. (2023b,a) (solid blue). The dotted black line shows the case when C/H scales linearly with O/H. The filled blue circles show the [C/O]−[O/H] pairs for which we performed the PDR modeling. The corresponding values for a linear relation between C/H and O/H are shown as empty blue circles. The observational data of Berg et al. (2016, 2019); Ravindranath et al. (2020); Izotov et al. (2023) are shown for comparison, including star-forming galaxies with abundances measured via recombination lines (Esteban et al. 2002, 2009, 2014; Pilyugin & Thuan 2005; García-Rojas & Esteban 2007; López-Sánchez et al. 2007). The secondary y-axis shows the log₁₀(C/O) abundance ratio. Right panel: $\mathcal{D}$–[O/H] relation for the dust-to-gas ratio normalized to 10⁻². The diamonds show observations from the DGS (Rémy-Ruyer et al. 2013) and KINGFISH (Rémy-Ruyer et al. 2014) surveys. The solid blue line shows the best-fit relation from these observations. The dotted black line shows the linear relation between the dust-to-gas ratio and O/H. As in the left panel, the empty and filled blue circles show the input values to the PDR models. The secondary y-axis shows the non-normalized dust-to-gas mass ratio.