The world itself is becoming much smaller by using modern traffic and modern communication. Life today is much easier than it was hundreds of years ago, but it has brought new problems. One of the biggest problems is pollution.
    To pollute means to make things unfit or unclean to use. Pollution comes in many ways. We see it, smell it, drink it and even eat it. Pollution is beginning to threaten (威胁) our health, our happiness and our life.
    Man has been polluting the earth from the time he first made fire, washed his clothes in the river and threw his waste on the ground. When land was used up or water became dirty, men moved on to another place. At first, the problem was not so serious because there was plenty of clean air, land and water. There weren't so many people then and their wants fewer. All the dirty things could be absorbed(吸收) by nature and soon covered over. But this is no longer true. The increase of pollution and the development of industry(工业) have changed that. Man is slowly poisoning(毒害)not only his environment but also himself.
    Through the use of poison, man has polluted the land, killing the animals. By putting dirty water and chemicals into rivers and lakes we have polluted our drinking water, killing the fish.
Our increasing population is part of the problem. More people, more waste.
     Where is this all to end? Are we turning the world into a big rubbish dump(堆存处)，or is there any hope that we can get rid of (摆脱，除去) the pollution? Luckily, millions of people have been warned of the danger of pollution. Large numbers of people are now working hard to bring pollution under control.
    The earth is our home. We must take care of it. This means keeping the land, air and water clean. And we must take care of the rise in pollution at the same time.

    Height is just one of the features your genes(基因) decide. In fact, because you have two parents, your genes provide you a height that usually lands somewhere between the height of each parent. If both your parents are tall, then most probably you will be tall, too. But if you have questions about how tall you're going to be, ask your doctor if he or she can help you find them out.

    But genes don't decide everything. For example, eating an unhealthy diet can keep you from growing to your full potential(潜力). Getting plenty of sleep and enough exercise will help you grow to the expected height.

    It's natural that you're wondering how fast you should grow. There's no perfect or right answer. Generally speaking, kids grow about 6 cm a year between age 3 and the time when they start puberty.

    Your doctor will know how your growth has been going over the years. 2 cm here and 3 cm there make a little difference to the height you're at now. How have you been growing up to this height? What other changes may your body be going through?

    Don't be amazed if you seem to have grown a lot in a very short time. Everyone has a growth spurt(高峰) during puberty. The age for starting puberty is about 10 for girls and about 11 for boys. But it can be earlier or later — between 7 and 13 for girls and 9 and 15 for boys.

    You'll usually begin to notice that you're growing faster about a year after your body starts to show the first changes of puberty.

根据材料内容选择最佳答案。

    Rik-rrork. That's the sound made by frogs singing at night. You might have thought the noises were just the musical sounds of ure. You probably didn't realize that you were listening to frogs' communicating with one another. These familiar sounds are not random(随意的) but are actually meaningful to frogs.

Being Heard but Not Seen

    Frogs are good at being heard but not seen. These animals have several characters that protect them n predators (捕食者 ). One is their size. Most frogs are small, which helps them hide easily. Many frogs have coloring that allows them to hide in their environment, making it difficult for predators to find them. In addition, most frogs are active at night. So it is difficult for most animals to see them in the dark.

Small but Loud

    Frogs call to tell other frogs where they are. Their voices can be very loud. But when frogs feel the footsteps of a larger animal, they become silent.

    Many of the world's frogs live in rain forests, where there are many different animals calling to their own species (物种 ) in their own ways. Frogs need to call loudly enough to be heard through the noise in order to find other frogs.

Singing with Purpose

      Peter Narins is a scientist who studies how frogs call to one another. He has learned that frogs use their calls to help them tell friend from enemy. They sing the same notes at the same volume (音量 ) each time they call. In addition, Narins has found that frogs of the same species always sing the same patterns (模式 ). These patterns are like a code that is different from other animals' calls and difficult for others to copy. Narins also observed that timing plays a role in frog calls. Many frogs call only at a certain time of night.

More Work to Do

    Scientists want to better understand the purpose of frog calls. Studies continue to explore the details of frogs' unique communication. Being heard but not seen is one way frogs stay safe. The idea that they have found a way to survive in a dangerous world is simply amazing.

    There are some easy things you can do to protect the environment and the earth. Choose ideas from the list or come up with a few of your own.

    ·Plant flowers, grass or trees.

    ·Whenever you visit a park or beach, take away what you bring there-keep rubbish in a bag until you can put it in a dustbin.

    ·Turn off the lights, TV sets when you leave the room. This saves a lot of electricity.

    ·Turn off the tap when you brush your teeth. You can save some water by not letting it run. Also, use a glass cup instead of a paper cup because this saves paper.

    ·Keep the doors and windows closed in winter to keep cold air in.

    ·Give your old books and magazines to a library instead of throwing them away.

    ·Give your old clothes to poor children you know instead of throwing them away.

    ·Use both sides of paper.

    ·Stop pouring dirty water into the rivers or lakes nearby.

    ·Encourage all your friends to do the same things you do to help protect the earth.

    You don't have to wait until Earth Day to do these things. Make every day Earth Day.

    If everyone makes a contribution to protecting the environment, the world will become much more beautiful.

    What's sleepwalking(梦游)？Sleepwalking usually happens in the first few hours of sleep. Not all sleepwalkers walk when they are sleeping. Some just sit up or stand on the bed. Some even get up and walk around for a few minutes or half an hour. Some do things as if they were awake, but in fact they are not!

    Sleepwalkers' eyes are open and they usually go back to beds by themselves. But they won't remember it the next morning.

    About 15% of kids sleepwalk. Sleepwalking sometimes happens when a person is ill or worried, or doesn't get enough sleep. If sleepwalking happens often, you should see a doctor. If it doesn't, you don't need to worry. Here are some ways to help stop sleepwalking:

    Listen to some music and relax before going to bed.

    Go to bed at a regular time every night. keep your bedroom quiet and dark when you try sleeping.

    Don't drink a lot of water in the evening and be sure to go to the bathroom before going to bed.