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:The endoglucanase, cellobiohy drolase, xylanase, and mannanase activities were determined using CMC, MCC, glucuronoxylan, and galactomannan (Sigma) as the substrates, respectively, by the initial speed of formation of reducing sugars (RS) by the Somogyi Nelson method [20–22].
Category: