1. Allen, I. E., & Seaman, J. (2006). Making the grade: Online education in the United States, 2006. Retrieved November 3, 2007, from The Sloan Consortium Sloan-C Web site. .
2. The concept of activity;Ashton,1996
3. The role of grounding in collaborative learning tasks;Baker,1999
4. Speech genres and other late essays;Bakhtin,1986
5. Unlearning to teach mathematics;Ball;For the Learning of Mathematics,1988