中国“八一”飞行表演队在某次为外宾做专场飞行表演时，飞机向上拉升做后空翻动作的一段轨迹可看成一段圆弧（如图）．设飞机做后空翻动作时，在最高点附近做半径r=160m的圆周运动．如果飞行员质量m=70kg，（g取10m/s²）．求：

   Let me tell you my pen friend Ivan. He is 12 years old. He likes playing football. He usually play football with his friends at weekends.
Ivan lives with his father and mother in a city in France. They have a dream home with five rooms, a swimming pool and a garden. They often have a party in the garden at weekends. They like to help their neighbours. His home is not near his school, so Ivan takes a bus for an hour and then walks for 10 minutes to school. There is a Basketball Club near his school. He likes play basketball in the club.
   Ivan's father is a worker. He is busy. Ivan's mother is a doctor. She is nice to the patients. Ivan wants to be a teacher when he grows up. I want to be a teacher, too. Every month Ivan writes two letters to me. He'll come to Suqian next year. We'll meet then.

A．纳米镍粒子的质量分数

B.碳原子的数目

C.氢元素的化合价

D.____