Funding for this research was provided by:
Ministry of Education of the People's Republic of China (16YJAZH015)
National Natural Science Foundation of China (71874185)
National Bureau of Statistics of China (2016LZ13)
the Natural Science Foundation of Jiangsu Province (Grant No. BK20170268)
Article History
Received: 23 May 2019
Accepted: 6 November 2019
First Online: 16 December 2019
Compliance with ethical standards
:
: The authors declare that they have no conflict of interest.