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Authors
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Funding
Funding for this research was provided by:
National Natural Science Foundation of China (81301799)
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Article History
Received: 14 January 2017
Accepted: 26 February 2018
First Online: 7 March 2018
Ethics approval and consent to participate
: All data were extracted from published articles and only population level data were included in this study, for this type of study formal consent is not required.
: No applicable.
: The authors declare that they have no competing interests.
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