求英语大神帮我解析一下这篇英语阅读

求英语大神帮我解析一下这篇英语阅读
I like taking photos very much. Once I travelled by ship ___1___ a cloudy morning. It was in the afternoon ___2___ the weather turned fine. Therefore I wanted to take some photos. I was looking around and I saw next to me a lovely little girl ___3___ sit on her father s shoulders. ___4___ was luchy for me to meet such a perfect moment—the girl was smiling and the surrounding view was beautiful.
Without thinking I took a picture ___5___ the little girl at that moment. Her mother was surprised that a ___6___ strange was photographing her daughter. I explained that I was moved by that sight and showed them ___7___ photo on my digital数字的 camera. They loved it very much. I said I could email it to them when I got home.
Finally they gave ___8___ their email address. The next day I sent them their daughter s photo as I ___9___ promise. This morning I received a message back saying they are grateful and ___10___ they are very happy with the lovely photo of their daughter and a lovely memory of the day.



答案
1. on 2. that 3. sitting 4. It 5. of 6. stranger 7. the 8. me 9. had promised 10. that




求大神帮我解析一下答案解析,为什么要这么填?