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:Both DG and DGS have been studied as an additional ethanol source by enzymatic hydrolysis of the residual polysaccharides (Bals et al., 2006; Kim et al., 2008b; Tucker et al., 2004).
Category: