Final answer:
Longer working hours is not a reason for increased world populations. Better healthcare, improved nutrition, and cleaner water have all contributed to rising life expectancy and population growth.
Option B.
Step-by-step explanation:
The option that was NOT a main reason why world populations have increased is B) Longer working hours.
Key factors that have contributed to increased populations include better healthcare, such as the development of immunizations and antibiotics; more food and better nutrition.
Allowing urban populations to be much better nourished than in the past; and cleaner water through improved systems for water supply and waste disposal, which have helped to prevent the transmission of many diseases.
These factors have led to an overall rise in life expectancy and a decrease in mortality rates, especially from infectious diseases.