What is the meaning of word "prevalence" in the sentence "Changing lifestyles play a key role in the increasing prevalence of obesity. "

A．Calorie is used to measure the energy used by the body.
B．Scientists are clear about which genes affect human obesity.
C．Obesity is determined by a person's biological factors.
D．Less time is devoted to exercise for both adults and children now.

A．expend the same amount of calorie as in taking
B．do many types of physical activities
C．exercise regularly
D．pay attention to diet