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:pPGI is strongly inhibited by light [21] and by 3-phosphoglycerate (3PGA) [22], a Calvin-Benson cycle intermediate accumulating in the chloroplast during illumination that allosterically activates AGP [19,20].
Category: