On my first day of high school, I met David. It was cloudy. When I went home after school, I saw a boy also walking back home. He was my classmate, David. He was carrying his books. 1 he was walking, I saw a group of kids rushing to him. David 2 and his glasses went flying. All his 3 fell on the ground. Then the kids ran away. He looked up and I saw sadness in his eyes. I helped him stand up and 4 his glasses. Then I saw there 5 a big smile on his face.
From then on, we became 6. The more I got to know David, the more I liked him. During the four years, we always studied, played together and had a 7 time.
On the day when we finished high school, I could see that he was 8 about his speech. So I said, “Don't be stressed out. Believe in 9 and you'll be the best!” Then he began, “Today is a good time to say 10 to those people who helped you succeed through the difficult years. Our parents, our teachers, but mostly our 11. Being a friend to someone is the best gift you can give them. I'll 12 you a story…”I just looked at the handsome popular boy amazingly when he told the story of the 13 day when we met each other.
All of us were 14. Everyone was listening to his story in silence. I saw his parents looking at me and smiling. I did not know the deep meaning 15 that moment. I know even a small action can change a person’s life. As you know, “Friends are angels to make us rise when our wings have problems flying.”
=7.8×103 kg／m3）制造，耗钢130吨；若采用“气凝胶”代替钢材来制造一架同样大小的 飞机，则需“气凝胶”的质量为 ( )