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 in low-income nations, is one reason for the low standard of living.

Here are the answers:
1. Population destiny is the number of people living in a square mile.
2. In the future, economically developed nations may not have enough money to take care of older citizens because of rapid population growth.
3. In low-income nations, ave life expectancy is one reason for the low standard of living.

Environmental factors