Department of Basic Courses, College of Basic Science, Ningbo Dahongying University, Ningbo, China
College of Economics and Management, Zhejiang University of Technology, Hangzhou, China
Copyright © 2017 Ying Zheng, Zhiqing Meng et al. This is
an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.