Volunteers ride bikes to promote low-carbon travel in Guilin city, South China's Guangxi Zhuang autonomous region, Sept 21, 2013. With the year's theme "Green Transport & Clean Air", 153 Chinese cities have taken part in World Car Free Day campaign and people are encouraged to take public transport, ride bikes or walk. The campaign first originated in France in 1998 and initiatively launched in China in 2007, aiming to promote environment-friendly transport and ease the city's traffic congestion. [Photo/Xinhua]