燕子每年秋天都要从北方到南方过冬，鸟类科学家发现，两岁燕子的飞行速度v与耗氧量x之间满足函数关系v=alog₂ $\frac{x}{10}$ ．若两岁燕子耗氧量达到40个单位时，其飞行速度为v=10m/s，则两岁燕子飞行速度为25m/s时，耗氧量达到____单位．

如图甲所示，在xOy平面内有足够大的匀强电场．电场方向竖直向上，电场强度E=40N/C，在y轴左侧平面内有足够大的磁场，磁感应强度B₁随时间t变化的规律如图乙所示，15π s后磁场消失，选定磁场垂直纸面向里为正方向．在y轴右侧平面内还有方向垂直纸面向外的恒定的匀强磁场，分布在一个半径为r=0.3m的圆形区域（图中未画出），且圆的左侧与y轴相切，磁感应强度B₂=0.8T，t=0时刻，一质量m=8×10^﹣4kg、电荷量q=+2×10^﹣4C的微粒从x轴上x_p=﹣0.8m处的P点以速度v=0.12m/s 向x轴正方向入射．（g取10m/s²）

阅读理解

    My school stood in a big square playground in southeastern South Dakota. One teacher taught all grades, first through eighth. Most grades had only two or three students.

    Our school day started with the flag pledge(宣誓). Then the teacher called one grade at a time to the recitation bench beside her desk. She'd check our work, explain the new lesson, and dismiss us to go back to our own desks and do our new work, all in less than ten minutes per grade.

    At noon we ate lunches we had brought. Our lunches consisted of homemade sandwiches and if we were lucky, dessert. My favorite dessert was a fresh pear, and a piece of Mom's delicious sour cream chocolate cake.

    The annual Christmas program was the most exciting part of the year. We hurried through our lessons during December to allow time to practise poems, songs, and plays.

    A few days before the performance, the school board members borrowed equipment from the town and set up a stage across one side of the classroom. We hung bed sheets for curtains.

    On the evening of the performance, petrol lanterns hanging along the walls cast a warm, though not very bright, light over the gathering crowd. We could hardly contain our excitement as we looked from behind the curtains to wave at our parents.

    On a spring Sunday in a new term, just before the last day of the school term, everyone in the neighborhood gathered for a picnic. Our moms set fried chicken, bowls of salads, and desserts on the teacher's desk and the library table. After the dinner, we played games. One of the school board members brought big buckets of ice cream in the afternoon to top off the picnic. How we looked forward to that treat!

    I was just nineteen years old when I started my first teaching position in a country school with thirteen students. I felt excited, nervous and happy as I prepared my lunch bucket the first morning of the term. I can't remember what kind of sandwiches I packed, but I do remember I put in a fresh pear and a piece of chocolate cake for dessert!