Why Has Our Contemporary Lifestyle,with Many People Spending Hours Sitting

Why has our contemporary lifestyle,with many people spending hours sitting sedentary in front of televisions or computers,contributed to a significant rise in obesity?

A) Because low muscle tone promotes insulin insensitivity
B) Because blood glucose levels remain stagnant during inactivity
C) Because leptin is most effective when people are active
D) Because humans use energy very efficiently,and less activity causes us to use less energy