Funding for this research was provided by:
National Natural Science Foundation of China (61374054, 61263002)
Fundamental Research Funds for the Central Universities (31920160003, 31920170141)
Program for Young Talents of State Ethnic Affairs Commission (SEAC) of China (2013-3-21)
the educational reform project for graduate courses of Northwest Minzu University
Article History
Received: 1 November 2017
Revised: 1 January 2018
Accepted: 12 January 2018
First Online: 20 June 2018