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:Among the available alternatives, we have selected three well-known tools: cbmc [27], a tool based on Bounded Model Checking (BMC), CPAchecker [15], a tool based on lazy abstraction and interpolation-based refinement, and Clousot [40], a recent tool based on Abstract Interpretation.
Category: