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:C33-I (Auersperg, 1964), HeLa (Puck et al., 1958), CaSki (Patillo, 1977) and SiHa (Friedl et al., 1979) cells were grown in Dulbecco's modified Eagle's (DME) medium supplemented with 10~ foetal calf serum.
Category: