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:Huett et al. (2005) compared nitrogen and phosphorus removal from plant nursery runoff in vegetated SSF CWs and unvegetated filters in Australia, and reported that removal efficiency for planted wetlands increased from 63.4 and 69.1% for TN and NO3–
Category: