The evolutionary track as predicted by models with various amounts of metal enhancements. The left two panels are log(C/O) vs. 12 + log(O/H) and [O/Fe] vs. [Fe/H]; the right two are oxygen abundance vs. gas fraction and Y vs. (O/H). All 3 models have the same burst sequence (t = 1/3/5/7/9/11/13 Gyr, and d = 0.1 Gyr for each burst), but different degrees of metal enhancement. The solid black lines, dashed red lines, dotted blue lines represent w_H,He = 0.1,0.5,1, respectively. All these models assume the same total infall mass M_inf = 10⁹ M_⊙. The observational data are the same as in Figs. 2–4, BCDs, dIrrs, and DLAs are plotted in open cyan circles, open cyan triangles, and open green squares here.