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:Color M-mode recordings were guided by the two-dimensional jet appearance in order to orient the interrogation plane through the zenith of the proximal convergence isovelocity area; this minimizes effects of errors due to the angle between the interrogation axis and the main axis of the jet (15).
Category: