What three factors influenced the gains in life expectancy between 1900 and 1974?

The correct answer emphasizes critical elements that have historically contributed to significant improvements in public health and life expectancy. Focusing on sanitation, nutrition, and chemotherapy reflects key advancements and practices that have collectively played a vital role.

Sanitation improved notably during this period with the implementation of sewage systems and clean drinking water standards, which drastically reduced the incidence of infectious diseases. Improved nutrition, through better access to food and a greater understanding of dietary needs, further enhanced public health, leading to better overall physical health and resilience against disease.

Chemotherapy, while more specific than general advancements in healthcare, highlights the impact of medical breakthroughs in treating previously deadly diseases, particularly bacterial infections and later, cancer treatments. These advancements combined represent a triad of factors that contributed directly to increased life expectancy during the 20th century.

This understanding reflects an intersection of public health initiatives, advancements in medical science, and social improvements that have reshaped mortality rates and overall population health.