Left: ratios ℛ of the modeled line ratios for [C i] 370 μm/[C i] 609 μm, [O i] 63 μm/[C ii], [O i] 145 μm/[C ii], [C ii]/CO(3–2), [C ii]/CO(1–0), and [C ii]/[C i]370 μm, for simulated plane-parallel clouds of different sizes with G_UV = 1 × 10³ and P = 3 × 10⁵ cm^-3 K. Right: same for spherical clouds of different radius. The vertical lines indicate the range of radius where the predictions of the model are compatible with the observed [C ii]/CO(3–2) and [C ii]/[C i]370 μm.