Submitted on: 24 March 1998.
Revised on: 30 April 1998.
Accepted on: 25 May 1998.
Physical Review Physics Education Research
http://reply.papertrans.cn/3/247/24637/24637-22.png
Submitted on: 10 November 2008.
Revised on: 18 December 2008.
Accepted on: 28 December 2008.
Physical Review Physics Education Research
http://reply.papertrans.cn/3/247/24637/24637-24.png
http://reply.papertrans.cn/3/247/24637/24637-25.png
http://reply.papertrans.cn/3/247/24637/24637-26.png
http://reply.papertrans.cn/3/247/24637/24637-27.png
http://reply.papertrans.cn/3/247/24637/24637-28.png
http://reply.papertrans.cn/3/247/24637/24637-29.png
http://reply.papertrans.cn/3/247/24637/24637-30.png