Line luminosity of the ¹²CO J = 10–9 emission lines, red crosses, versus their bolometric luminosity, together with the line luminosity of the ¹³CO J = 10–9 spectra, blue diamonds, multiplied by the assumed abundance ratio of ¹²C/¹³C for the entire WISH sample of YSOs. The line luminosity of the C¹⁸O J = 10–9 lines, green triangles, multiplied by the assumed abundance ratio of ¹⁶O/¹⁸O is plotted together with the previous values. The dashed line represents the linear fit of the ¹²CO J = 10–9 spectra, the full line that for the ¹³CO J = 10–9 transition and the dash-dot line indicates the fit for the C¹⁸O J = 10–9 data.