Temporal evolution of selected species and D/H ratios during the cloud (top panels) and core stages (lower panels). Panels (a) and (e) show the abundances of selected species during the cloud and core stages, respectively. Solid, dashed, and dotted lines denote models with Z = 1, 0.5, and 0.2, respectively. The remaining panels show the temporal evolution of the D/H ratios and the H₂ OPR for Z = 1 (b), (f), Z = 0.5 (c), (g), and Z = 0.2 (d), (h). In all panels, time is normalized by τ_freeze for the corresponding density and metallicity.