【正确答案】[参考样文]
Dear Mrs. Benton,
I'm so sorry to have to write you this letter! After accepting your kind invitation for dinner on Tuesday, May the fifth, I now find that I cannot be there after all.
I have just been called to Shanghai on business. Although I tried hard to put off the trip until after May the fifth, I was unable to do so. The Office Head told me that the business tour was quite urgent. A company in Shanghai threatened to discontinue buying the products we make. I was required to negotiate with it about the matter.
I had been looking forward with great pleasure to seeing you, and I now feel very bad about this unexpected development. I hope that you understand me and that we'll meet in the near future.
Sincerely yours,
Li Ming

【答案解析】Notes: after all 毕竟，终究。on business 出差。put off 推迟。discontinue 中断。feel very bad about 感到抱歉。