Funding for this research was provided by:
National Natural Science Foundation of China (81071656, 81272318)
Text and Data Mining valid from 2019-03-18
Received: 15 January 2019
Accepted: 13 March 2019
First Online: 18 March 2019
Ethics approval and consent to participate
: This study was approved by the Ethics Committee of Hunan Normal University.
: All authors reviewed and approved the manuscript.
: The authors declare no conflict of interests.
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