根据短文内容，选择正确答案。
B
    Nick is my pen pal. He is 12 years old. He is from Canada. He lives in Ottawa. He speaks English and French. His favorite subject is science. He likes Chinese, too. And he says he can speak a little Chinese, but he can't write in Chinese.
    Nick likes sports, too. He likes swimming best. He goes swimming on weekends with his father. He says he can swim very well. He doesn't like physics. He thinks it's too difficult.
    Nick has a big family. There are nine people in his family, his grandpa, grandma, father, mother and his brothers and sisters. He has two brothers and two sisters. They are Bob, Tim, Jane and Susan. They are happy!

--- Who told you my address ? --- I don’t remember clearly. It _____ Li Mei.
　 ---It _____ Li Mei. She doesn’t know it.

某同学利用气垫导轨做“探究碰撞中的不变量”的实验；气垫导轨装置如图所示，所用的气垫导轨装置由导轨、滑块、弹射架、光电门等组成．

⑴下面是实验的主要步骤：

①安装好气垫导轨，调节气垫导轨的调节旋钮，使导轨水平；

②向气垫导轨通入压缩空气；

③接通光电计时器；

④把滑块2静止放在气垫导轨的中间；

⑤滑块1挤压导轨左端弹射架上的橡皮绳；

⑥释放滑块1，滑块1通过光电门1后与左侧固定弹簧的滑块2碰撞，碰后滑块1和滑块2依次通过光电门2，两滑块通过光电门后依次被制动；

⑦读出滑块通过两个光电门的挡光时间分别为滑块1通过光电门1的挡光时间△t₁=10.01ms，通过光电门2的挡光时间△t₂=49.99ms，滑块2通过光电门2的挡光时间△t₃=8.35ms；

⑧测出挡光片的宽度d=5mm，测得滑块1（包括撞针）的质量为m_l=300g，滑块2（包括弹簧）质量为m₂=200g；

⑵数据处理与实验结论：

①实验中气垫导轨的作用是____②碰撞前滑块1的速度v₁为____m/s；碰撞后滑块1的速度v₂为____m/s；滑块2的速度v₃为____ m/s；（结果保留两位有效数字）

③在误差允许的范围内，通过本实验，同学们可以探究出哪些物理量是不变的？通过对实验数据的分析说明理由．（至少回答2个不变量）．

a．____b．____．

化简 $\frac{2x+2y}{5a^{2}b}\cdot \frac{10a{b}^2}{x^2-y^2}$ 的结果为____．

下列数组中，是勾股数的是（  ）