Funding for this research was provided by:
National Social Science Foundation of China (14BRK027)
Received: 1 May 2018
Accepted: 18 January 2019
First Online: 28 January 2019
Ethics approval and consent to participate
: This study was reviewed and approved by the Ethics Committee of Beijing Jiaotong University (Project No. 14BRK027). Informed consent in written form was obtained from legal guardians before the survey.
: Not applicable.
: The authors declare that they have no competing interests.
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