Footracing is a popular activity in the United States. It"s not only seen as a competitive sport, but also as a way to exercise, to enjoy the comraderies (同志情谊) of like - minded people, and to donate money to a good cause. Though serious runners may spend months training to compete, other runners and walkers might not train at all. Those not competing to win might run in an effort to beat their own time, or simply to enjoy the fun and exercise. People of all ages, from those less than one - year old (who may be pushed in strollers) to those in their 80s, enter into this sport. The races are held on city streets, on college campuses, through parks, and in suburban areas, and are commonly 5 to 10 kilometers in length.The largest footrace in the world is the 12 - kilometer Bay to Breakers race that is held in San Francisco every spring. This race begins on the east side of the city near San Francisco Bay and ends on the west side at the Pacific ocean. In 1993 there were 80, 000 people running in this race through the streets and hills of San Francisco. In the front are the serious runners who compete to win, and who might finish in as little as 34 minutes. Behind them are the thousands who take several hours to finish. In the back of the race are those who wear costumes just for fun. In 1993 there was a group of men all of whom dressed up like Elvis Presley. There was a group of firefighters who were all tied in a long line and were carrying a firehouse. One year there was a bridal (新婚的) party in which the bride dressed in a long white gown and the groom wore a tuxedo (晚礼服). They threw flowers to bystanders, and were actually married at some point along the route.The main purpose of this passage is to ______.
A．encourage people to runB．describe a popular activityC．make fun of runners in costumeD．give reasons for the popularity of footraces