http://info.biz.hc360.com/2007/12/05072368327-3.shtmlhttp://hi.baidu.com/%CC%F1%B5%AD%CE%AA%C0%D6/blog/item/994117f0db8063c17931aaac.htmlhttp://www.wwenglish.com/t/d/jijin3/listenessay/16992.htm这几个介绍的挺不错的,你可以学习一下
Dear Sir/ Madam