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:This could be attributed to interindividual differences in the bioavailability of ingested pyrene, extent of pyrene transformation to 1- OHP, and polymorphism of drug-metabolizing enzymes (Viau et al. 2002).
Category: