Funding for this research was provided by:
National Natural Science Foundation of China (81700452)
Received: 24 June 2018
Accepted: 22 November 2018
First Online: 4 December 2018
Ethics approval and consent to participate
: This study did not require approval by the local Research Ethics Board as it involved publicly available data only.
: Not applicable.
: The authors’ declare that they have no competing interests.
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