【正确答案】[范文]
Dear Mr. Zhang,
   I would like to express my apologies to you for not being able to keep our 10 o'clock appointment.
   The reason why I didn't keep the promise is that I witnessed a traffic accident on my way to school. The victim was a school girl. She was hit by a car from the back and fell down her bicycle. She was badly injured and lay unconsciously on the side of the road. It was annoying that the car driver just ran away after hitting the girl. Several kind-hearted passersby and I eventually took the poor girl to the hospital.
   I will come over to your apartment the day after tomorrow, Tuesday, Feb. 26th, at 9 o'clock in the morning if that is convenient for you. Sorry again.