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 efficient removal of lignin due to the prior steam pretreatment and the lower acid content of the cooking liquor likely limited the decrease in the molecular weight of the lignin in the pulping liquor which is typically observed during organosolv pretreatment (Pan et al., 2006a).
Category: