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:Other forms of elevated CK signaling, such as CK application or overexpression of IPT [39, 40] or KNOTTED genes [41], were insufficient to promote tubers in tomato, probably due to inappropriate local hormonal balances or to changing ratios among CK metabolites [42].
Category: