What is the sky？ Where is it？ How bright is it？ What lies above it？ These questions are difficult to answer, aren’t they？

　　Is the sky blue？ The sky has no color. We know that there is air around the world. When planes fly, they need air to lift their wings. Planes can’t fly very high because the air gets thinner when they go up higher. If we go up about 1,200 kilometers from the earth, we’ll find no air there.

　　When we talk about the sky, we usually mean the atmosphere around the earth. It is like a very thick “cover” over the earth and keeps it warm. Without the “cover”, the earth would be a very cold place and nothing could live on it.

　　Looking up above us, you will see the blue sky with white clouds. It is usually light blue, but the sky would be dark blue if you go to the top of the highest mountains. And you will see fewer clouds between you and the sun.

　　And if you sit in a spaceship high above the earth, there is no air at all. The sky would be so dark that its color would be black.

　　Sometimes the sky doesn’t look blue. When the sun rises and falls, you will see red, orange, and other colors in the sky. These are the most beautiful moments of the day.

　　Now you may understand that we see colors because the air scatters (散射) the sunlight. Light travels through the air. Without air, there would be no light and no colors.

1.Planes can’t fly very high because ____________.

A．there is air around the world

B．the air gets thinner when they go up higher

C．it’s warmer in the sky

D．the air gets thicker when they go up higher

2.We can see colors of the sky because ____________.

A．the sun rises and falls every day

B．the sky has lots of colors

C．the air scatters the sunlight

D．the sunlight has many colors

3.Which of the following is TRUE？

A．There is little air without light and colors in the sky.

B．The sky will look red, orange and other colors at noon.

C．The sky has only one color when the sun rises and falls.

D．The sky is dark blue if you go to the top of the highest mountains.

 

Computers can injure you. Most other injuries happen suddenly. For example, if you fall off a bike and break your arms, it happens very quickly. But computer injuries slowly.
　 You probably know how to ride a bike safely. Now learn to use a computer safely.
Your eyes
　 Too much light can injure your eyes, so never sit too close to a computer screen. Your eyes should be at least 50 centimeters from the screen. Remember to look away from it sometimes. This gives your eyes a rest.
　 When you use a computer, the window should be on your left or your right. If it is behind you, the light will reflect on (反射) the screen. If the window is in front of you, the sun and the screen will both shine into your eyes.
Your hands and wrists (手腕)
　　Hand and wrist injuries can happen because the hands and wrists are moved in the same way hundreds of times. If you use a keyboard for a long time, follow these three rules: 1) Rest your wrists on something. 2) Keep your elbows (肘部） at the same height as the keyboard. 3) Stop something and exercise your hands, wrists and fingers in a different way.
Your back
　　Some people sit for many hours in front of a computer. If you sit in the wrong way, you can injure your back or your neck. So you should sit with your back straight. The top part of the screen should all be in front of your eyes. Your forearms（前臂）, wrists, hands and the upper（上部） part of your legs should all be parallel (平行) to the floor. If you are sitting for a long time, get up every 30 minutes and exercise your arms, legs and necks.
　 Enjoy your computer, but use it safely.
【小题1】A computer screen may injure your eyes if __________.

A．you sit 60 centimeters away

B．you remember to look away

C．your eyes are too close to it

D．the window is on your left or your right

【小题2】Hand and wrist injuries are caused when you _________.

A．get plenty of exercise

B．move in the same way again and again

C．rest your wrists on something

D．keep your elbows as high as the keyboard

【小题3】Your eyes and hands will be safe if __________.

A．you work near a window

B．you often take a rest from working

C．you use a keyboard for a long time

D．the window is behind you

【小题4】It is good for your back and neck if you sit __________.

A．in the right way

B．for many hours

C．with the screen below your eyes

D．with your back leaning forward

【小题5】What would be the best title for the passage?

A．How to protect your eyes

B．How to protect your hands and wrists

C．How to use a computer safely

D．How to protect your back