According to Dunbar,why Would Larger Social Groups Be Associated with the Evolution

According to Dunbar,why would larger social groups be associated with the evolution of larger brains?

A) Larger groups provided more protection for survival,allowing for evolution of larger brains.
B) Smaller groups tended to be too cohesive and unwilling to adopt new ideas,leading to stagnant brain evolution.
C) Smaller groups were more vulnerable to predation and defeat in warfare,preventing the evolution of larger brains.
D) Larger groups were more successful in hunting,and the additional food led to the evolution of larger brains.
E) Larger groups had greater social complexity,driving the evolution of larger brains to handle such complexity.