在不计空气阻力的情况下，下列运动中加点物体机械能守恒的是（   ）

单词拼写 根据所给汉语或英语首字母写出短文中所缺英语单词的正确形式。
      Last week, my friends and I talked about the rules in our school. Here are some of our ideas.
      At our school, we have to wear uniforms every d____ . The problem is that all my classmates think the uniforms are  丑的____ .We think young people should l____  smart and so we would like to wear our own clothes. Our teachers believe that if we did that, we would concentrate more on our clothes than our studies. We don't 同意____ .We would feel more comfortable and that is g____  for studying. If we can't do that, we should be 允许____  to design our own uniforms. That would be a good way to keep both teachers and 学生____ happy.
      Besides, vacations should be longer. At present they’re too s____ . Longer vacations would give us time to do things like volunteering. Last summer I had a(n)  机会____  to volunteer at the local hospital,b____  I couldn’t because I had to go back to school.