The gains in life expectancy between 1900 and 1974 occurred mostly due to __________.

The significant gains in life expectancy between 1900 and 1974 can be attributed to multiple interrelated factors, with sanitation, nutrition, and education all playing crucial roles.

Sanitation improvements, such as the development of sewage systems and access to clean drinking water, drastically reduced the transmission of infectious diseases. The implementation of public health measures, including waste disposal and hygiene practices, contributed to lower mortality rates, particularly in urban areas.

Nutrition also had a substantial impact on life expectancy during this period. Better agricultural practices, food distribution systems, and dietary awareness led to improved nutrition, which is vital for maintaining health and preventing diseases. As populations had greater access to a variety of foods, deficiencies and malnutrition-related diseases declined.

Education is another key component. A more educated populace is generally better informed about health practices, nutrition, and hygiene. Increased awareness about preventive measures, healthcare access, and healthier lifestyles further contributed to the improvements in public health.

These factors synergistically contributed to the increased life expectancy during that time period, making the option that includes all of them the most accurate representation of the reasons behind these gains.