Funding for this research was provided by:
Beijing Social Science Fund (17SRB005)
Received: 28 August 2018
Accepted: 28 May 2019
First Online: 7 June 2019
Ethics approval and consent to participate
: The study was approved by the Ethical Review Committee of the Capital Medical University(No. 2018SY95 and No.2018SY96). Participation in the survey was completely voluntary and written consents were obtained from participants.
: Not applicable.
: The authors declare that they have no competing interests.