1. School of Economics, Qingdao University, Qingdao, Shandong, China
2. School of Economics, Nankai University, Nankai, Tianjin, China
3. School of International Economics and Trade, Nanjing University of Finance and Economics, Nanjing, Jiangsu, China
4. Nanjing Institue of Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, Nanjing, Jiangsu, China