A、To run as fast as they can. B、To take a long miming race. C、To keep running without stopping. D、To make sure they are healthy

Today, millions of people run in the United States. Many more women now take part in the sport. Many children in public and private schools run as part of their physical education programs.
     (16)Running has gained popularity for several reasons. You can do it anywhere, any time. You do not need other people. And you do not need much equipment. However, experts say you should wear a good pair of running shoes to protect your feet. The manufacture of running shoes has become a huge industry.
    People run for different reasons. Most say running makes them feel better physically. It prevents them from gaining weight. It provides needed exercise to help prevent some diseases.(17)Many people also say running makes them feel better mentally. It makes them feel happier. Some say they forget their worries when they run.
     (18)Many people also run to help others. For example, Ed Burt of Hopedale, Massachusetts ran in the Boston Marathon to help the American Liver Foundation. This deeply pleased his father, who was suffering from liver disease.
    Sports experts urge people to prepare themselves before trying to run in long races. They say special exercises and repeated runs are needed to build strength.
     (19)Doctors also urge runners to make sure they are in good health before entering a running race. They say 42 kilometers is a long way to run as fast as you can, without stopping.
16. Why is running becoming popular?
17. How does running help people become better mentally?
18. Why does Ed Burt run?
19. What is doctors’ advice for runners?