基金supported by the National Natural Science Foundation of China(No.3146028631660307+7 种基金31260252)the Natural Science Foundation of Xizang(Tibet) Autonomous Region(No.2015ZR-13-192016ZR)Innovation program of University Young Teacher,Tibet Autonomous Region(QC2016-29)the Social Science Foundation of the Chinese Ministry of Education(No.12YJA850011)the School Foundation of Xizang Minzu University(No.11myY20)Xizang Minzu University graduate practice and innovation project in 2016(No.2016MDYJS001)