如图所示，一个带正电荷的小球沿水平光滑绝缘的桌面向右运动，飞离桌子边缘A，最后落到地板上．设有磁场时飞行时间为t₁，水平射程为x₁，着地速度大小为v₁；若撤去磁场，其余条件不变时，小球飞行时间为t₂，水平射程为x₂，着地速度大小为v₂.则下列结论正确的是(　　)

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    Running away is a serious problem. There are lots of runaway and homeless kids living in the streets around the world.

①孩子们为什么想离家出走？

    Some kids run away because they're afraid to tell their parents the bad things that they did at school or in society.

②Some kids run away because there is violence(暴力）in the family.

    Some kids run away because their parents divorce(离婚）or marry again.

    Some kids run away because they have problems at school.

    What will you do if your friend wants to run away from home?

    If your friend is thinking about running away, tell him or her that it is difficult to survive(幸存）in the streets. Try to help your friend feel less alone.

    ③Whatever the problem is, there are other ways to solve it. Maybe you can't think of the ways right now, but an adult(成年人）will know how to help you. You should tell an adult that your friend plans to run away as soon as possible. Being a real friend doesn't mean keeping a secret that may hurt someone. And running away isn't a solution(解决方法）for either of you.64.It only makes more problems and danger.

利用化学知识，完成下列相关问题：
①在氮气、氧气、二氧化碳、二氧化硫气体中，体积百分含量为21%的气体是____，造成“酸雨”的气体是____。（均用“化学式”表示）。
②水是人类宝贵的自然资源。
I．净水器中，常用____（填“名称”）作吸附剂除去水中的色素和异味。
II．打开汽水瓶塞，有气泡冒出，说明气体溶解度与____有关。
III．水电解时，连接电源负极的玻璃管内的气体具有的性质是____（填序号）。
A．有色有味　　　　B．能够燃烧　　　　C．支持燃烧　　　　D．易溶于水
IV．下图是水通电后分解过程中微粒变化的部分示意图，请按它们在化学变化过程中出现的顺序排列____（用图中的字母表示），反应的化学方程式是____。

V． 个水分子中含有____mol原子。

掌握仪器的名称、组装及使用方法是中学化学实验的基础，根据如图实验装置回答问题．

如图所示，一个弹簧振子沿光滑的杆在水平方向做简谐运动，振子的质量为0.2kg，当它运动到平衡位置左侧2cm时，受到的回复力为4.0N．当它运动到平衡位置右侧4cm时，它的加速度大小和方向为（   ）