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:Cell viability results showing that lung cancer cells exhibit resistance to the cytotoxicity of high concentration of ATP in comparison to normal cells are consistent with published data that showed low (nearly undetectable) expression levels of P2X7R in lung cancer cells (20, 21, 39).
Category: