Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Given the abundance of fine ash particles in the volcanic plume, it is thought that cloud condensation nuclei are readily available (Woods 1993; Williams and McNutt 2005; Durant et al. 2008) and the model assumes condensation occurs immediately once the gaseous phase becomes saturated with respect…
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