题干

下列关于空气及其成分的说法正确的是(   )

A:氮气是空气中含量最高的气体,可以用于食品防腐等

B:氧气占空气含量的21%,可用于急救病人

C:稀有气体充入霓虹灯,通电能发出不同颜色的光,这是稀有气体的化学性质

D:实验室用氯酸钾和二氧化锰加热制氧气,二氧化锰是催化剂,提高了氧气产量

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A

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    Developing good study habits isn't easy.____ In this way, you'll become more and more successful as a student.
    Start by allowing no interruptions____You have to give 100 percent of your attention to your work. Don't allow yourself to be distracted by friends or fellow students. Keep your goal in mind; know what you have to do and when it needs to be done. Don't let other people influence you into not doing your work.
    ____If you study best in the morning, plan to do your hardest subjects first. Remember not to leave today's work till tomorrow. A delay like this is called procrastination. If you delay for any reason, you'll find it more and more difficult to get things done when you need to. You may rush to make up the time you wasted getting started, resulting in careless and error-filled work.
Another good way to study is to take part in partnered and group-study sessions. These allow students to share ideas, and help each other deal with difficult material____
    Forming good study habits can be difficult at first, and requires energy and a strong will, But if you commit yourself to it, success will be yours____By doing so, you will know that you have tried. If you are satisfied with what you've done and know that you have tried your best, your grade will be the icing on the cake.
A. Always give your best in the classroom and in study sessions.
B. It's impossible to study while chatting or watching TV.
C. Figure out when you're most productive.
D. You may do well enough but maybe you can hardly achieve your goal easily.
E. Putting off your task for any reason can make it difficult to finish on time.
F. But start with a plan, put in a lot of hard work, and studying will become easier.
G. In addition, learning among students is a good way of getting a firmer grasp of the material.