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:Costunolide (1) has been studied various inhibitory activities of melanogenesis (Choi et al., 2008), production of tumor necrosis factor-alpha and interleukin6 (Pae et al.
Category: