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:It has been generally accepted that planted wetlands outperform unplanted filters, and there is a positive correlation between plant uptake and nutrient removal from agricultural treatment wetlands (Comin et al., 1997; Huett et al., 2005; Gottschall et al., 2007).
Category: