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:100 million years ago (mya), and duplication of the alcohol dehydrogenase encoding gene and genes encoding hexose transporters [4,11] have been proposed as a possible molecular background for development of Crabtree effect and the ‘‘makeaccumulate-consume’’ strategy in the S.
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