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:For non-dominated sorting, we also used for comparison the three state-of-the-art algorithms, namely, Best Order Sort [20], the k-d tree-based algorithm called ENS-NDT [9], and the hybrid algorithm combining the divide-and-conquer idea [2] and the ENS algorithm [26] similar to how this was done in this paper.
Category: