1. Lecturer, Dept. of Economic Management, North China Electric Power Univ., Hebei 071003, People’s Republic of China.
2. Ph.D. Degree Candidate, School of Economics and Management, North China Electric Power Univ., Beijing 102206, People’s Republic of China (corresponding author).
3. Professor, School of Economics and Management, North China Electric Power Univ., Beijing 102206, People’s Republic of China.