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:Similar results in vertebrates were found in a study of fries of the fish Clarias gariepinus Burchell, 1822 (Siluriformes: Clariidae) in which the effects of hydrocarbons (crude oil, benzene, toluene, and xylene) were examined after 4 days of exposure (Otitoloju & Olagoke 2011).
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