Population density is the number of people living in a square mile. In the future, economically developed nations may not have enough money to take care of older citizens because of underpopulation. In low-income nations, overpopulation is one reason for low standard of living.
The fact that US citizens have a high per capita income (per capita = per person) allows them to consume a disproportionately large amount of products compared to the rest of the world. So I think the answer is B.