假设你是新华中学的学生李华，你和在上海上学的英国朋友Tom约好下周末去北京旅游，但你因故不能赴约。请根据以下要点用英语给他写一封电子邮件：
1. 表示歉意；
2. 解释原因；
3. 另约时间。
注意：1. 词数120~150；
2. 可适当增加细节。
范文：
Dear Tom,
It's a long time since we met last time. I miss you very much. How”s everything going? I”m writing to say sorry to you because I can't obey our promise. Don”t be angry. Please give me one minute to explain it.
In fact, my mother asked me to visit my grandma that day. You know grandma is to me what water is to fish. I had a good memory with her. And we should spare no effort to take care of the old, shouldn't we?
Friend, you're still young but my grandma is old! Taking these factors into consideration, I decided to write the letter finally. 作文批改系统，易优作文批改、作文点评、作文打分系统，<a href=https://www.euzW.net/jiaoshoulanmu/>易优作文批改</A>
Let's make it next Saturday. I think you can understand me. Now, say sorry again, looking forward to your reply.
