Funding for this research was provided by:
National Natural Science Foundation of China (71371141)
Received: 22 November 2018
Accepted: 6 August 2019
First Online: 13 August 2019
Ethics approval and consent to participate
: The needs for ethics approval and informed consent were unnecessary according to the national guidelines promulgated by the National Health and Family Planning Commission [CitationRef removed].
: Not applicable.
: The authors declare that they have no competing interests.