Funding for this research was provided by:
Peking University (BMU2017YB005)
Received: 8 March 2019
Accepted: 29 July 2019
First Online: 5 August 2019
Ethics approval and consent to participate
: This study was granted an exemption from requiring ethics approval by the Peking University Institutional Review Board because the survey was anonymous and did not include sensitive questions. An introduction about the survey was provided on the first page of the questionnaire, including aims, the main contents of this survey and promise to keep the data anonymous and confidential. The sampled students had the right to withdraw at any time.
: Written proof of exemption from research ethics review was obtained from Peking University. A copy of the proof is available for review by the Editor of this journal.
: The author(s) declared no potential conflicts of interest concerning the research, authorship, and publication of this article.