According to the passage, what was the percentage of American adults doing regular physical exercises two years ago ( )

A．research showed their health benefits
B．people got the message from magazine covers and postage stamps
C．scientific evidence of health benefits was shown on TV ads
D．an increasing number of races were held in American cities

A．are yet to be proved
B．are to be further studied
C．are supported by scientific evidence
D．are self-evident