In a certain year, the number of tourists in five cities including Jicheng, Yucheng, Xucheng, Wucheng, and Qingcheng was more than in previous years. These cities have different geographical environments and tourism resources: steep mountains, dense forests, flat Grassland, vast sea, and quiet river valley. The number of tourists (persons / year) to the five cities mentioned above were: 120,000, 270,000, 320,000, 440,000, and 650,000. Known: (1) Cities in the grassland are the least attractive to tourists. The most popular tourist destination is the forest. (2) Xu Cheng is in the mountains and the transportation is not convenient, but there are many people who come here. (3) The ratio of tourists to Jicheng Gaocheng (4) Gaocheng has 440,000 tourists, a record high (5) Yucheng has more tourists than Qingcheng, but not as much as Xucheng. According to the above conditions, the characteristic tourism resources that can be launched in Qingcheng are--. ['Secluded river valley.', 'Dense forest.', 'Flat grassland.', 'The vast sea.']