http://reply.papertrans.cn/3/233/23246/23246-31.png
http://reply.papertrans.cn/3/233/23246/23246-32.png
Submitted on: 23 August 2011.
Revised on: 29 September 2011.
Accepted on: 22 October 2011.
North American Journal of Economics and Finance
http://reply.papertrans.cn/3/233/23246/23246-34.png
http://reply.papertrans.cn/3/233/23246/23246-35.png
http://reply.papertrans.cn/3/233/23246/23246-36.png