What is the first thing you want to do when you see someone yawn(打哈欠)? You want to yawn, too! Yawning is contagious.

    Scientists have found that all people yawn. Babies start to yawn even before they are born. The earliest yawn takes place when a baby is still inside its mother. When young children are about one year old, they start to yawn when they see others yawning. That's why scientists seem to think yawning is contagious.

    It's no secret that yawning is contagious, but why do we yawn? Well, scientists are not really sure.

    Some think we yawn because we feel bored. Recently scientists studied two groups of students between the ages of 17 and 19. One group watched music videos, and the other watched a boring color test. Scientists compared the numbers of their yawns and found that the color test group yawned more times than the video group.

    Another reason about yawning has to do with breathing. Some scientists believe that when we are bored or tired, we breathe more slowly. As our breathing slows down, we cannot get rid of(去除) enough carbon dioxide (CO₂) in our bodies. When there is too much carbon dioxide, our bodies will tell the brains (大脑) to breathe deeply. The result is a big yawn.                                                           

    In 2007, scientists suggested a new reason. They said that the reason people yawned was to cool down their brains. These scientists found that people who were warmer than others yawned more often.

    Scientists are still trying to find out why yawning happens and why it is so contagious. But they are only sure that when one person yawns, almost everyone else nearby wants to yawn, too.

下列四个图案中，属于中心对称图形的是（   ）

根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

    Taking good notes is a time-saving skill that will help you to become a better student in several ways ____.Second, your notes are excellent materials to refer to when you are studying for a test. Third, note-taking offers variety to your study time and helps you to hold your interest.

    You will want to take notes during classroom discussions and while reading a textbook or doing research for a report. ____ Whenever or however you take notes, keep in mind that note-taking is a selective process. ____

    The following methods may work best for you.

●Read the text quickly to find the main facts and ideas in it.

●Carefully read the text and watch for words that can show main points and

supporting facts.

● Write your notes in your own words.

●____

● Note any questions or ideas you may have about what was said or written.

    As you take notes, you may want to use your own shorthand(速记). When you do, be sure that you understand your symbols(符号) and that you use them all the time. ____

A．Use words, not complete sentences.

B．There are three practical note-taking methods.

C．You must write your notes on separate paper.

D．Otherwise, you may not be able to read your notes late

E. you will also want to develop your own method for taking notes.

F. That means you must first decide what is important enough to include in your notes.

G. First, the simple act of writing something down makes it easier for you to understand and remember it.

下列物质：①氧化铝；②空气；③镁粉；④氧气；⑤氧化铜；⑥液态空气；⑦经过过滤后的矿泉水；⑧冰水混合物。其中属于纯净物的是____(填序号)；属于单质的是____(填化学式)；属于化合物的是____(填化学式)；属于氧化物的是____(填化学式)。