下列物质按单质、氧化物、混合物的顺序排列的是

对豆科植物和根瘤菌的共生关系描述不正确的是（ ）

    Taking a math test can be pretty stressful. Even if you know the material, you can still get the problem wrong. ____ Therefore you'll improve your grades, as well as your math skills.

    Write it out

    You can also check a math problem by writing everything out on paper. ____ Writing out math problems reduces your chances of missing anything to the lowest possible level, which is a common cause of incorrect answers.

    ____

    Make sure your answers work by doing the opposite procedure of what your problem calls for, including the answer you got the first time around. In other words, you would use the opposite of this addition problem—subtraction (减法)—to determine whether your answer is the correct one.

    Plug in（代入）

    You may find that a variable isn't good enough or have a problem where you have to solve for a variable (变量). ____ This is the only real way to assure yourself that the answer you've found is correct.

    ____

    If the result of a problem seems to make no sense, it indicates that the answer is incorrect. For example, if you get an answer in the millions and you know it should be in the thousands, you've likely misplaced a point. Go back through the work on your paper to make sure all of your formulas and calculations are correct. If everything looks okay, do the problem again and compare the result of the second try to what you've got on the first try.

A. Do the opposite.

B. Correct the answers.

C. Plug the variable in the equation (方程) to check it out.

D. Knowing how to go through your math test and checking your work can save you from handing in a test full of mistakes.

E. It also helps you to figure out everything after you have already finished the test.

F. Check for a reasonable answer

G. This helps you to know what information you have and what information you need to solve.

阅读下文，回答相关问题。

                                                                                                                             一个美丽的故事
        有个塌鼻子的小男孩，因为两岁时得过脑炎，智力受损，学习起来很吃力。打个比方，别人写作文能写二三百字，他却只能写三五行。但即便这样的作文，他同样能写得美丽如花。
        那是一次作文课，题目是《愿望》，他极认真地想了半天，然后极认真地写，那作文极短，只有三句话：我有两个愿望，第一个是，妈妈笑眯眯地看着我说：“你真聪明”第二个是，老师天天笑眯眯地看着我说：“你一点也不笨。”
        于是，就是这作文，深深地打动了他的老师，那位妈妈式的老师不仅给了他最高分，在班上带感情朗诵了这篇作文，还一笔一画地批道：你很聪明，你的作文写得非常感人，请放心，妈妈肯定会格外喜欢你的，老师肯定会格外喜欢你的，大家肯定会格外喜欢你的。
        捧着作文本，他笑了，蹦蹦跳跳地回家了，像只喜鹊。但他并没有把作文本拿给妈妈看，他是在等待，等待着一个美好的时刻。
        那个时刻终于到了，是妈妈的生日——一个阳光灿烂的星期天。那天，他起得特别早，把作文本装在一个亲手做的美丽的大信封里，信封上面画着一个塌鼻子的男孩儿，那小男孩儿咧着嘴笑得正甜。他静静地看着妈妈，等着妈妈醒来。妈妈刚刚睁眼醒来，他就甜甜地喊了声“妈妈”，然后笑眯眯地走到妈妈跟前说：“妈妈，今天是您的生日，我要送给您一件礼物。”
        妈妈笑了：“什么？”
        他笑笑：“我的作文。”说着双手递过来那个大信封。
        接过信封，妈妈的心在怦怦地跳！
        果然，看着这篇作文，妈妈甜甜地涌出了两行热泪，然后一把搂住小男孩儿，搂得很紧很紧，仿佛他会突然间飞了。
        是的，智力可以受损，但爱永远不会，它朝气蓬勃，永远垂着绿阴，开着明媚的花，结着芳香的果。