【正确答案】Dear Mr. Smith,
I feel greatly honored to be invited to your Christmas party to be held on the evening of December 24th. 2010. I truly wish that I could spend the wonderful evening with you. Unfortunately, I cannot do so. I am very sorry.
I have been busy with my preparation for the National Entrance Examination for MA/MS Degree during the past several months. The exam is to be held on next January 8th.Therefore. time is so limited for me. I am afraid that I have to work hard day and night.
What a shame! However, I would like to express my best wishes in advance. I wish you a Merry Christmas and a Happy New Year!
　　　　　　　 　　　　 Yours truly,
　　　　　　　　 　　　　　 Li Ming