LVG results for the [ CI ] and ¹²CO line flux ratios observed in the galaxy sample. The panels contain lines of constant flux ratio, as a function of H₂ gas kinetic temperature T_k and volume density n(H₂), for distinct C and CO column densities N/dV. From left to right: a. intersection of curves of constant line ratio [ CI ] (2−1)/(1−0) = 1.5 (≈20 K) and corresponding line ratios ¹²CO (4−3)/¹²CO (2−1) = 4.5 and ¹²CO (7−6)/¹²CO (4−3) = 0.9; b. intersection of curves of constant line ratio [ CI ] (2−1)/(1−0) = 2.0 (≈25 K) and corresponding line ratios ¹²CO (4−3)/¹²CO (2−1) = 6 and ¹²CO (7−6)/¹²CO (4−3) = 1.05; c. intersection of curves of constant line ratio [ CI ] (2−1)/(1−0) = 2.5 (≈30 K) and corresponding line ratios ¹²CO (4−3)/¹²CO (2−1) = 7 and ¹²CO (7−6)/¹²CO (4−3) = 1.35. In all three panels, CO curves are given for column densities N(CO)/dV = 1 × 10¹⁷ cm^-2/ km s^-1, (upper curve), 3 × 10¹⁷ cm^-2/ km s^-1, and 1 × 10¹⁸ cm^-2/ km s^-1 (lower curve). Since the single [ CI ] (2−1)/(1−0) curve represents a column density N(CO)/dV = 1 × 10¹⁷ cm^-2/ km s^-1, the CO curves effectively correspond to [C]/[CO] abundances of 1, 0.3, and 0.1, respectively.