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:A growing number of animal studies showed that imaging or therapeutic agents larger than the BBB’s exclusion threshold of 400 Da could be successfully delivered by FUS with microbubbles (Alonso et al. 2013; Choi et al. 2007; Choi et al. 2010a; Hynynen et al. 2001; Kinoshita et al. 2006; Marquet et al. 2014; Shen et al. 2014).
Category: