Funding for this research was provided by:
National Natural Science Foundation of China (71272150)
Key Projects of the National Social Science Research Fund of China (15AZD012)
Social Science Research Fund of Inner Mongolia Autonomous Region in China (2017NDC145)
Article History
Received: 7 November 2019
Accepted: 18 February 2020
First Online: 24 March 2020
Competing interests
: The authors declare that they have no competing interests.