supported by the National Natural Science Foundation of China(No.31460286;31660307;31260252;);the Natural Science Foundation of Xizang(Tibet) Autonomous Region(No.2015ZR-13-19;2016ZR);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)