Histogram showing the distribution of the ratio (indicated by black data points) between the linewidths of CO and H₂O. The H₂O linewidth are from Y16 and O13. It is clear that most of our sources have similar CO and H₂O linewidth, with an average ratio of 1.0 ± 0.1. For G09v1.97, the IRAM-30 m spectrum shows that the asymmetric CO line is ~1.5 times larger than that of the H₂O line (Y16) as shown by the dashed error bar and histogram. However, using the ALMA high S/N CO and H₂O spectral data of G09v1.97 (Yang et al., in prep.), we find a ratio of the linewidth to be very close to 1 as indicated by the red point. The source of SDP 81 is not shown here because it is a special case as discussed in the text.