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 particularly useful tool in demonstrating the carrier mediation of this phenomenon was the use of previously injected urea analogs ([14C]-acetamide, [14C]-thiourea), which were “pulsed” simultaneously to urea at different rates (Wood et al. 1998; McDonald et al. 2000).
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