Funding for this research was provided by:
The Educational Science Foundation of Shaanxi of China (18JK0830)
Scientific research plan projects of Xianyang Normal University of China (14XSYK003)
Article History
Received: 4 November 2019
Accepted: 21 January 2020
First Online: 3 February 2020
Competing interests
: The author declares to have no competing interests.