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:Various pretreatment techniques, such as hot water (Kim et al., 2008b), dilute acid (Lloyd and Wyman, 2005), lime (Kim and Holtzapple, 2005), ammonia fiber expansion (AFEX) (Teymouri et al., 2005), ammonia recycle percolation (Kim and Lee, 2005) have been investigated thoroughly.
Category: