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:…used in the present study has been proven to be suitable for the extraction of resveratrol from several transgenic plants (Szankowski et al., 2003; Zhu et al., 2004; Rühmann et al., 2006), resveratrol has not been found in methanol or acetone extracts of strawberry (Aharoni et al., 2002;…
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