W31C. A two-component ALI model of the hot core emission at V_LSR = −3.5 km s^-1 (marked with a vertical dashed line), and foreground envelope absorption at V_LSR = 0 km s^-1. The absorptions from interstellar gas along the sight-lines at V_LSR = 11−50 km s^-1 are not modelled. The relative difference of the modelled and observed continuum at the frequency of each line is given in the legend. The resulting ortho-to-para ratio for this model is 2.6 in the envelope.