School of Agricultural Economics and Rural Development, Renmin University of China, Beijing 100872, P.R.China;
School of Agricultural Economics and Rural Development, Renmin University of China, Beijing 100872, P.R.China;
School of Agricultural Economics and Rural Development, Renmin University of China, Beijing 100872, P.R.China;
School of Agricultural Economics and Rural Development, Renmin University of China, Beijing 100872, P.R.China;
School of Continuing Education, Renmin University of China, Beijing 100872, P.R.China;
School of Agricultural Economics and Rural Development, Renmin University of China, Beijing 100872, P.R.China;