Concept:Rapid population growth occurs when birth rates remain high while death rates fall sharply.
Explanation:Improved social and medical facilities reduce mortality, especially among infants and mothers.
Better healthcare controls diseases and increases life expectancy.
Better social amenities improve living standards and encourage larger families to survive.
A stable government does not directly cause rapid population growth.
Rainfall and its reliability affect agriculture and settlement, but not global population growth directly.
The type of agricultural system practised does not determine the speed of world population growth.
Therefore, the key factor is improved social and medical facilities.
Answer:A. improved social and medical facilities