Funding for this research was provided by:
National Natural Science Foundation of China (21663030)
The Project of Science & Technology Office of Shaanxi Province (2015GY174)
The Project of Science & Technology of Yan’an City (2016CGZH-12-01)
Article History
Received: 3 September 2018
Accepted: 29 December 2018
First Online: 7 January 2019