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:Efficient phytoextraction species should translocate high quantities of heavy metals (HMs) into their aboveground biomass without toxicity symptoms, and at the same time produce large amounts of biomass (Vangronsveld et al., 2009; Luo et al., 2016).
Category: