Funding for this research was provided by:
NSAF Joint Fund (11671029)
the Project for Youth Scholars of Higher Education of Hebei Province (QN2017402)
the Project of Teaching and Research of Beijing Jiaotong University Haibin College (HBJY16005)
Article History
Received: 23 August 2018
Accepted: 2 December 2018
First Online: 13 December 2018
Competing interests
: The authors declare that there is no conflict of interests regarding the publication of this paper.