【单选题】

According to the passage, initial improvement in brain function in Homo's ancestors resulted at least partially from which of the following
A．(A) the sharing of resources typical of hunter-gatherer groups to which Homo eventually came to belong

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