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:Several papers have found that past subsidies significantly expanded demand for solar systems (Hughes and Podolefsky, 2015; Burr, 2016; De Groote and Verboven, 2016; Gillingham and Tsvetanov, 2017).
Category: