Improved nutrition, medical treatment, sanitation, and safety have contributed to the dramatic gains in average in the united states in the last 100 years. functional age chronological age life expectancy brain age

Average Life Expectancy

Explanation:

According to my research and based on the information given within the question, I can say that these categories made a huge difference in the Average Life Expectancy in the United States in the last 100 years. This is because these are all categories that help you maintain your body healthy thus adding years to a persons life. About 100 years ago the life expectancy was an average of 40 years and now in 2019 it is an average of 75 years in the US.

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b) overload, progression, specificity, regularity, recovery, balance, and variety