1. Keegan, D., 1986. The foundations of distance education, London: Croon Helm.
2. Moore, M. G., Cookson, P., & Donaldson, J., 1990. Contemporary issues in American distance education, New York: Pregamom Press.
3. The U.S. Department of Education, from http: /www. ed. gov/index. jhtml.
4. Burder, I., 1989. Distance learning: What's holding back this boundless delivery system?, Electronic Learning, 8(6): 30-35.
5. The Distance Education and Training Council (1999). from http: /www. detc. org.