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:Pretreatment of the bone samples followed a modified Longin (1971) procedure to extract collagen for 14C measurement, while pretreatment of the mollusk shells involved a 20% removal of the outer shell surface by etching in 1M HCl (Ascough et al. 2005).
Category: