反应方程下方的数字是中子及有关原子核的静止质量（以原子质量单位 u 为单位，取1u 的质量对应的能量为9.3×10² MeV，此裂变反应中（   ）

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    Grammar is the system of a language. People sometimes describe grammar as the “rules” of a language; but in fact no language has rules. If we use the word “rules”, we suggest that somebody created the rules first and then spoke the language, like a new game. But languages did not start like that. Languages started by people making sounds which evolved (逐渐发展成) into words, phrases and sentences. No commonly-spoken language is fixed. All languages change over time. What we call “grammar” is simply a reflection (反映) of a language at a particular time.

    Do we need to study grammar to learn a language? The short answer is “no”. Very many people in the world speak their own, native language without having studied its grammar. Children start to speak before they even know the word“grammar”. But if you are serious about learning a foreign language, the long answer is “yes, grammar can help you to learn a language more quickly and more efficiently.” It's important to think of grammar as something that can help you, like a friend. When you understand the grammar (or system) of a language, you can understand many things yourself, without having to ask a teacher or look in a book.

    So think of grammar as something good, something positive, something that you can use to find your way---like a signpost(路标) or a map.

    Except invented languages like Esperanto（世界语）. And if Esperanto were widely spoken, its rules would soon be very different.

下列各句中，划线成语使用恰当的一项是（    ）

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Never try to work when you are very hungry. If you decided to do your homework right after school, you may get something to eat before getting to work. Always do your homework before you get too tired. Don't wait until very late in everything, or your work will seem much harder than it really is.

    If you have more than an hour's work, give yourself a break after an hour. On the other hand, don't break it up so much, or you can't finish anything, you should be able to work at least a half at a time without stopping.

    Don't put it off until the last minute. If you put off doing your homework, you will have it on your mind, and you will not enjoy your free time so much. If you put it off until the end of the week or until right before a test, you will have too much work to do. A little bit each night, enough to keep up with what is happening each day in school, will make you not afraid of the test and get good grades.

Do your homework at the same time every evening. This will help you make it a habit. It will make it easier to do, and it will make your free time more enjoyable, as well.