RADEX modeling of the [C II] line emission with (red lines) and without (blue lines) FIR background caused by high dust opacity in the quasar host galaxy, and compared with the observed [C II] line emission (black circles with error bars): the [C II] surface brightness profile (left panel), the [C II] line equivalent width (middle panel), and the ratio between Σ_[C II] and Σ_FIR (right panel) as a function of radius. The dashed black line in the right panel represents the radial distribution of the optical depth at the frequency of the [C II] underlying dust continuum.