Final answer:
Since 1900, 4. life expectancy in the United States has indeed increased by over 30 years, from an average of 47 years to about 79 years by 2015. While some events have caused temporary decreases, the general trend has been a significant rise in life expectancy due to public health improvements and advancements in healthcare.
Step-by-step explanation:
The correct statement regarding life expectancy in the United States since 1900 is that life expectancy has increased by over 30 years since then. In the early 20th century, the average life expectancy was about 47 years, while by 2015, it had risen to approximately 79 years.
This significant increase is attributed to various factors, including advancements in healthcare, better living standards, and effective public health programs. Despite slight declines due to events like the Great Influenza of 1918 and recently the COVID-19 pandemic, which reduced life expectancy to 77 years in 2020, the general trend has been upward over the past century.
Historical events such as military conflicts, pandemics, and economic downturns have occasionally impacted life expectancy, causing fluctuations. However, the overarching trend has been an improvement in longevity, making the correct answer to the question: 4) Life expectancy in the United States has increased by over 30 years since 1900.