Beijing--No Car Day was first started by 34 cities in France on September 22, 1998. It was started to protect the environment. By now, more than 1,000 cities around the world have had a No Car Day. The first No Car Day in China was in Chengdu in 2001. Other cities, including Beijing, Taipei, Shanghai and Wuhan also support the day. In Beijing, more and more people are joining in the activity. It asks drivers to leave their cars for one day each week and walk, take a bus or subway or ride a bike to work. The slogan for the activity is,"If we drive for one less day, we can have one more nice day." Beijing is trying to have 238 blue sky days this year. In the first quarter of this year Beijing only have 52 blue sky days. This was 11 days less than the number for the same period last year. Car exhaust  is one of the most important reasons of the pollution. We must do more for No Car Day. So far, more than 200,000 drivers have shown their support. "We can't control the weather, but we can choose not to drive." said Wu Zonghua, a car club chairman. The air pollution in Beijing is mainly caused by  _  according to the passage.
cars dust weather rubbish