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:Reduction of the pentavalent to the trivalent forms can also be catalysed by GSTO1-1 or non-enzymatically (Cohen et al., 2006; Tapio and Grosche, 2006).
Category: