To Illustrate the Problem of the Collective Rate of Consumption

To illustrate the problem of the collective rate of consumption on sustainability, if everyone on Earth lived the lifestyle of a traditional Indian villager, it is arguable that even 12 billion would be a sustainable world population.If everyone lives like an upper-middle-class North American, the level of population above which it would be unsustainable is ______.

A) 2 billion
B) 4 billion
C) 6 billion
D) 8 billion