Over how many years did the research involving over 300,000 people occur?

The correct choice indicates that the research involving over 300,000 people took place over 13 years. This duration is significant as it reflects a comprehensive study that likely considered numerous variables and long-term health outcomes. When studies span several years, especially those involving large populations, they can provide more reliable and valid results. A longer study duration also allows researchers to observe changes over time, assess the impact of various factors, and contribute valuable data to the field of health care. Shorter timeframes might not capture the complexities and nuances of health trends that develop over a longer period, making 13 years a more suitable answer for comprehensive and impactful research in this context.