1. Xiao Li-ming (corresponding author), School of Economics and Management, Shanxi Normal University, Linfen 041004, Shanxi, P R China.
2. Jia Rui, School of Economics and Management, Shanxi Normal University, Linfen 041004, Shanxi, P R China.
3. Jing Rui, School of Economics and Management, Shanxi Normal University, Linfen 041004, Shanxi, P R China.