Spectra of observed E-type CH₃OH transitions towards W31C, excluding those in K-band (18–26 GHz), which are collected in Fig. 4. The panels from (a) to (l) are arranged following the order in Table A.1. The quantum numbers and rest frequencies of these transitions are labelled in their respective panels. Class I and II CH₃OH maser transitions are labelled in red and blue, respectively, and their Gaussian fitting results are plotted in the corresponding colour. The transitions that have no maser detection yet are labelled in black, and the Gaussian fitting results are plotted in green. In panel (k), the −30 km s⁻¹ component is caused by the blend of four CH₃OCHO transitions (i.e. 20_0,20 − 19_0,19 E, 20_0,20 − 19_0,19 A, 20_1,20 − 19_1,19 E, and 20_1,20 − 19_1,19 A). The vertical magenta dashed lines represent the systemic velocity of V_LSR = −3.43 km s⁻¹.