[Sun, S.] School of Software and Microelectronics, Peking University, Beijing, 100260, China
National Engineering Research Center for E-Learning, Central China Normal University, Wuhan, 430079, China
[Liu, J.] School of Electronics Engineering and Computer Science, Peking University, Beijing, 100791, China
National Engineering Research Center of Software Engineering, Peking University, Beijing, 100791, China
[Zheng, Z.] School of Software and Microelectronics, Peking University, Beijing, 100260, China, National Engineering Research Center for E-Learning, Central China Normal University, Wuhan, 430079, China