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:Two possible interpretations of these observations are (a) the spontaneous hydrolysis of PMSF (James 1978) and/ or (b) the existence of spontaneous reactivation (Estévez and Vilanova 2009).
Category: