Funding for this research was provided by:
Department of Education of Hebei Province (D20151706))
Text and Data Mining valid from 2018-03-07
Received: 14 September 2017
Accepted: 21 February 2018
First Online: 7 March 2018
: The authors declare that they have no competing interests.
: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.