Funding for this research was provided by:
National Natural Science Foundation of China (21675131)
Municipal Science Foundation of Chongqing City (CSTC-2015jcyjB50001)
National Undergraduate Training Program for Innovation and Entrepreneurship (2016106335036)
Article History
Received: 9 April 2018
Accepted: 24 April 2018
First Online: 8 May 2018
Compliance with ethical standards
: The author(s) declare that they have no competing interests.