【正确答案】[参考范文]
Dear Xiao Dong，
I am very sorry that I was out when you came to see me yesterday afternoon. The fact is that I had an engagement with some friends, and I was ignorant of your visit. Not until nine o' clock in the evening did I come back. You must have been disappointed by my absence.
I hope you will not leave the city this week. I'll call you on Friday afternoon at five o' clock. As this is the first time you come to Beijing, I will take you to some places that. may interest you.
Please wait for me in your hotel at the appointed time.
Yours truly,
Liu Ling