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:DBL1 (3D7 and 7G8), DBL1+2 FCR3, DBL2 FCR3, DBL3 (FCR3) and DBL5 (FCR3) were coupled at 1 mg/million microspheres [24]; FV2 was coupled at 3 mg protein/million microspheres based on previous optimization studies [23].
Category: