A、Because of their hobbies and pastimes. B、Because of their enthusiasm for sports. C、Because of their fear of heart attacks. D、B

In recent years, many Americans of both sexes and various ages have become increasingly interested in improving their bodies. They have become more devoted to physical fitness. Many persons have a strong desire to be more physically fit.
    By nature, Americans are enthusiastic, and energetic about hobbies or pastimes. And now many of them apply this enthusiasm, optimism, and energy to running. As a result, there are more running clubs to join and numerous books and magazines to read about running.
    The desire for American people to be physically fit could probably be explained by a "passion" for good health. The high rate of heart attacks in the 1960s has caused an increase of interest in improving the health condition of human body. Middle-aged men especially suffer from heart attacks. Thus, they are one group strongly interested in taking more physical exercise. In fact, many doctors often encourage their patients to become more physically active, especially those who have inactive jobs. It is quite interesting to note that the rate of heart attacks began to decrease between 1972 and 1974 and it is still decreasing now.
    It seems that physical fitness has now become a new "love" of Americans. But will it last very long? Nobody knows. Only time will tell—or until another "new passion" comes along and takes its place.
19. Why are Americans so interested in physical exercise?
20. What happens when American people apply great enthusiasm in running?
21. What kinds of patients, are especially encouraged to take part in physical exercise?