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