Solid lines show the predicted dependencies of the N v/C iv and C iv/He ii flux ratios on the carbon relative abundance [C/O]. The panels a), b), and c) show models with (n_H, U, Z_NLR/Z_⊙) = (10^1.0, 10^-2.0, 1.0), (10^4.0, 10^-2.0, 1.0), and (10^4.0, 10^-1.5, 0.5), respectively. The gray shadows denote the ranges that are consistent withe observational limits on the N v/C iv and C iv/He ii flux ratios in TN J0924−2201. The allowed [C/O] range is shown as the dotted line and arrow. The minimum carbon relative abundance ([C/O]_min) for each model is also given in each panel.