Top panels: Averaged emission-line flux ratios of low (blue), middle (purple), and high (red) mass subsamples plotted on the diagnostic diagram on C IVλ1549, He IIλ1640 and C IIIλ1909. The model predictions with different gas densities (log n_H = 1, 4, and 5) are presented in the left, middle, and right panels, respectively. Constant metallicity and constant ionization parameter sequences are denoted by orange and blue solid lines, respectively. The gray bars denote the root mean square of the data distribution and the estimated errors in the averaged values are shown as color bars. Bottom panels: NLR metallicities as a function of stellar masses, estimated with different three gas density models. Open circles would be less-likely solutions (see text).