基金
This work is supported by the Program of Humanities and Social Science of Education Ministry of China（nos.14XJC630003）, the Natural Science Foundation of China（nos.51408288）, the Natural Science Foundation of Gansu Province （nos.1308RJYA030）, and the Higher School Science Foundation of Gansu Province（nos.2014A- 044）. The authors wish to thank anonymous referees and the editor for their comments and suggestions.
