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:Wound healing of monolayers of human dermal fibroblasts was evaluated by a standardized scratch assay following a previous protocol with medium containing 10% FCS serving as a PC and medium containing 0% FCS serving as a NC [18].
Category: