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:Concentration data were requested for As, Cd, Cu, Fe, Mn, Ni, Pb and Zn, which represent the most common (and most commonly measured) pollutants discharged from abandoned metal mines in England and Wales (Mayes et al., 2009).
Category: