1. Ph.D. Candidate, School of Economics and Management, Beijing Jiaotong Univ., Beijing 100044, China. E-mail: xy72@yeah.net
2. Instructor, School of Civil Engineering and Architecture, Zhejiang Univ., Hangzhou 310027, China; and, Ph.D. Candidate, School of Economics and Management, Tongji Univ., Shanghai 200092, China. E-mail: rbfnnn@163.com