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:Urine in humans typically contains 10–30% inorganic arsenic, 10–20% MMA, and 60–80% DMA, indicating relatively efficient methylation (Cohen et al., 2006; NRC, 1999; Vahter and Concha, 2001).
Category: