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:The presence of multiple structural suites with unique characteristic wavelengths was interpreted to indicate a progressive thickening
of the layer with continued cooling [e.g., Hansen and Willis, 1998; Ghent and Hansen, 1999; Hansen et al., 2000].
Category: