根据短文理解，选择正确答案。
Winter-swimming has become popular in Beijing. Three years ago, few people would go swimming in the city waters. But now there is a Winter-swimming Enthusiasts' Club(冬泳爱好者协会)and it has more than 2,000 members. The oldest is 84 years of age and the youngest is only 7. The members are from all ways of life__They may be workers, peasants, soldiers, teachers, students...
Though it is now the coldest part of the season and the water temperature in the city's lakes is around 0°C, many winter-swimmers still swim in the icy waters, even when it is snowing. They enjoy themselves in the lake, while the people by the side of the lake are wearing heavy clothes.
Why are so many people interested in winter-swimming? Because winter-swimming can be good for one's health.
Bei Sha is a good example. He is 69 years old, and he once suffered from heart trouble for 26 years. After ten years of winter-swimming he is now in good health.
Scientists are now studying the effects of winter-swimming on health.
