[C II]/TIR intensity ratio versus TIR for the northern H II region BCLMP 691 in M 33. The blue drawn line shows the result of an unweighted linear least-squares fit to the TIR-bright data of BCLMP 691 varying only the y-intercept, while keeping the slope fixed at −1. The black dashed line shows the result of an unweighted linear least-squares fit to all M 33 data (cf. Fig. 5). A few typical 1σ errorbars are shown.