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 high concentration of Ca activates mitochondrial calcium-sensitive K channels (mtKca) and mitochondrial nitric oxide synthase (mtNOS), which increases the levels of nitric oxide (NO) (Figure 3) [21, 23, 33, 64, 100, 101, 107–115].
Category: