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:The ratio of mtDNA/NUMTs did not change with development from the base of the stalk to the leaf tip in dark-grown, light-grown, and dark-to-light-transferred seedlings, similar to our findings for maize ptDNA (Zheng et al. 2011).
Category: