Funding for this research was provided by:
National Natural Science Foundation of China (71872094)
Received: 4 January 2018
Accepted: 28 June 2020
First Online: 8 August 2020
Ethics approval and consent to participate
: Ethics approval is not applicable in the study.
: The survey respondents of the study have given their consent for the data to be used and published in this scientific article.
: The authors declare that they have no competing interests.