According to the passage, initial improvement in brain function in Homo's ancestors resulted at least partially from which of the following

A. the sharing of resources typical of hunter-gatherer groups to which Homo eventually came to belong
B. the freeing of Homo's heat-sensitive brain from the temperature constraints that had kept its size in check
C. the availability of foods far denser in calories and nutrients than those previously available
D. a rise in Homo's brain weight relative to his body weight
E. morphological solutions of environmental change developed by the robust proto-humans