Funding for this research was provided by:
Received: 13 June 2018
Accepted: 18 December 2018
First Online: 7 January 2019
Ethics approval and consent to participate
: The study was approved by the Ethical Review Committee of School of Public Health, Fudan University. Informed consent was obtained from guardians or parents prior to the survey.
: Not applicable.
: The authors declare that they have no competing interests.
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