What Happens to Physical Activity Through the 20s and 30s

What happens to physical activity through the 20s and 30s across developed countries?

A) Because of starting families, young adults are more physically active because they are likely playing with their children.
B) Due to starting careers, physical activity increases tremendously.
C) Physical activity continuously drops.
D) Since most jobs require considerable physical activity, young adults engage in increasingly more physical activity.