Ping, N
Sun, A
Song, Y
Wang, Q
Yin, J
Cheng, W
Xu, Y
Wen, L
Yao, H
Ma, L
Qiu, H
Ruan, C
Wu, D
Chen, S
Article History
Received: 17 January 2016
Revised: 28 April 2016
Accepted: 20 May 2016
First Online: 15 June 2016
Competing interests
: The authors declare no conflict of interest.