Police officers, fire fighters and medical staff hold a drill at a subway station in Nanjing city, capital of east China’s Jiangsu province, March 31, 2010. The Nanjing police held the exercise named "Zhongshan 2010" to test the city’s security system and emergency treatment before the World Expo opens. Nanjing is just 300 kilometers west of the Expo host city Shanghai. [Photo/Xinhua]