Funding for this research was provided by:
National Natural Science Foundation of China (11626079)
Natural Science Foundation of Hebei Province of China (F2015402033)
Scientific Research Project of Department of Education of Hebei Province of China (BJ2017031)
Article History
Received: 21 January 2018
Accepted: 10 June 2018
First Online: 22 June 2018
Compliance with ethical standards
:
: The authors claim that they have no conflicts of interest.
: This article does not contain any studies with human participants or animals performed by any of the authors.