Xu, G
Ji, C
Song, G
Zhao, C
Shi, C
Song, L
Chen, L
Yang, L
Huang, F
Pang, L
Zhang, N
Zhao, Y
Guo, X
Article History
Received: 7 October 2014
Revised: 6 May 2015
Accepted: 17 May 2015
First Online: 22 May 2015
Competing interests
: The authors declare no conflict of interest.