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

America is a country on the move. In unheard of numbers, people of all ages are exercising their way to better health. According to the latest figures, 59 percent of American adults exercise regularly, up 12 percent from just two years ago and more than double the figure of 25 years ago. Even non-exercisers believe they would be more attractive and confident if they were more active.
    It is not hard to get the message. The virtues of physical fitness are shown on magazine covers, postage stamps, and television ads for everything from beauty soaps to travel books. Exercise as a part of daily life did not catch on until the late 1960s when research by military doctors began to show the health benefits of doing regular physical exercise. Growing publicity for races held in American cities helped fuel a strong interest in the ancient sport of running.
    Although running has levelled off in recent years as Americans have discovered equally rewarding and sometimes safer forms of exercise, such as walking and swimming, running remains the most popular form of exercise.
    As the popularity of exercise continues to mount, so does scientific evidence of its health benefits. The key to fitness is exercising the major muscle groups vigorously enough to approximately double the heart rate and keep it doubled for 20 to 30 minutes at a time. Doing such physical exercises three or more times a week will produce considerable improvements in physical health in about three months.
Questions 16 to 18 are based on the passage you have just heard.
16. According to the passage, what was the percentage of American adults doing regular physical exercises two years ago?
17. What can be learnt about the health benefits of exercise from the passage?
18. Which would be the best title for the passage?