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:Heavy metals contamination of Bohai Bay in the past few years have been investigated (Zhang et al. 2002; Hua et al. 2005; Wang et al. 2005; Wang and Li 2006; Wang and Wang 2007; Liu et al. 2007; Meng et al. 2008), however, no comparison was available to show the spatial and temporal distribution of…
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