Submitted on: 22 May 2009.
Revised on: 18 September 2009.
Accepted on: 14 October 2009.
___________________AJIDD-American Journal on Intellectual and Developmental Disabilities
http://reply.papertrans.cn/1/12/1105/1105-32.png
http://reply.papertrans.cn/1/12/1105/1105-33.png
http://reply.papertrans.cn/1/12/1105/1105-34.png
http://reply.papertrans.cn/1/12/1105/1105-35.png
http://reply.papertrans.cn/1/12/1105/1105-36.png