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:In the subsequent studies, Yoshida et al. (2004b) and Ishii et al. (2004) confirmed the reversibility of 2,6-dihydroxybenzoate decarboxylase of Rhizobium sp. strain MTP-10005 and R.radiobacter WU-0108, respectively.
Category: