Funding for this research was provided by:
National Natural Science Foundation of China (81602914)
Received: 21 February 2020
Accepted: 24 April 2020
First Online: 6 May 2020
Ethics approval and consent to participate
: Not applicable; this analysis is based on published aggregate data and does not require ethical approval or informed consent.
: Not applicable.
: The authors declare that they have no competing interests.