Ratios of the observed line intensities to those predicted by the best-fitting LVG models as a function of upper level energy of the considered transitions towards L1551-IRS5. Upper panel: blue and red circles are for the ortho and para c-C₃H₂, respectively. Middle panel: Blue and red stars show the E and A transitions of CH₃OH, respectively. Bottom panel: blue and red diamonds are for the E and A CH₃CN lines, respectively.