In the last 50 years, the number of people with chronic disease in the united states has increased which of these statements best explain why this is true?
A) The population has increased in the united states during the last 50 years
B) The number of calories eaten per day has decreased in the united states during the last 50 years
C) the rate of physical activity has decreased in the united states during the last 50 years
D) the rate of physical activity has increased in the united states during the last 50 years​