Funding for this research was provided by:
Beijing Science and Technology Planning Project (D171100003117001)
Received: 12 June 2019
Accepted: 30 August 2019
First Online: 18 September 2019
Ethics approval and consent to participate
: Because this study was based on information retrieved from published studies, it did not require any ethical approval.
: Not applicable.
: The authors declare that they have no competing interests.