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:It is common for individual Streptomyces strains to have the capacity to both produce and resist inhibition by many antibiotic compounds (Challis and Hopwood, 2003; Davelos et al., 2004a; D’Costa et al., 2006).
Category: