To determine the main reason for the predicted increase in world population by another 4 billion people by 2100, we need to consider the age groups contributing to this growth. The question provides three potential reasons for the increase in population:
1. There will be more children (age below 15).
2. There will be more adults (age 15 to 74).
3. There will be more very old people (age 75 and older).
Let's analyze each option:
1. More children (age below 15): An increase in the number of children might suggest higher birth rates. However, significant increases in this age group are often tempered by global trends such as family planning, improved education, and economic factors that typically result in lower birth rates over time.
2. More adults (age 15 to 74): This age group constitutes the bulk of the population. Advances in healthcare, better living conditions, and increased life expectancy mean that more people will live to be adults and remain within this broad age range for longer periods. This increase supports a greater number of working-age adults and older but still active adults.
3. More very old people (age 75 and older): While there is indeed an increase in life expectancy and hence a growing number of elderly individuals, the provided prediction of 4 billion additional people is less likely to be predominantly due to this group alone, given that the elderly population, although increasing, still represents a smaller percentage of the total population compared to younger age categories.
Given our consideration, the main reason for the predicted 4 billion increase in world population by 2100 is that there will be more adults (age 15 to 74).
Thus, the correct answer is:
- There will be more adults (age 15 to 74).
