Line luminosity of C¹⁸O vs. ¹³CO (J = 2−1) (panel a)) and C¹⁸O vs. [C i] (J = 1−0) (panel b)) for ten models, run also with a reduced initial atomic C and ¹³C abundances. The standard value, X_C = [C]/[H] = 1.35 × 10^-4, has been reduced by factors of 3 and 10. The models with ISM-like carbon abundances are shown by the outlined symbols, those with a factor of 3 of carbon depletion by the filled symbols and those with a factor of 10 depletion by the unfilled symbols.