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 bottoms of the Bohai and Yellow Seas are composed mainly of sand, mud and mixed sediment and are not suitable for the
settlement of A. coerulea planula larvae (Chen & Zhu, 2012; Duarte et al., 2012).
Category: