LSR velocity as a function of the upper energy level (upper panel) and critical density (lower panel) for W31C. The red and blue dots indicate the methanol lines with absorption features and maser features, respectively. The black dots indicate the methanol emission without a narrow and strong maser emission feature. We only show the J₂ − J₁E lines with absorption features or maser features in the panels. Their velocities are derived from Gaussian fitting to the emission peaks or absorption dips. The error bars indicate Gaussian fitting errors. The horizontal magenta dashed line represents the systemic velocity of V_LSR = −3.43 km s⁻¹.