C

    Rivers are one of our most important natural resources(资源). Many of the world's great cities are located on rivers, and almost every country has at least one river through it that plays an important part in the lives of its people.

    Since the beginning of history, people have used rivers for transportation. The longest one in the United States is the Mississippi. The lifeline of Egypt is the Nile. To the people of India, the Ganges is great, but it is also important for transportation. Ships can travel along it for a thousand miles. Other great rivers are the Congo in Africa and the Mekong in southeastern Asia. The greatest of all for navigation(航海), however, is the Amazon in Brazil. It is so wide and so deep that large ships can go about two thousand miles up it.

    Besides transportation, rivers give food, water to drink, water for crops, and chances for fun and recreation for the people who live along their banks. In order to increase the supply of water for crops, engineers sometimes build a dam(坝) across a river and let a lake form behind the dam. Then people can use the water not only to irrigate(灌溉) their fields but also to make electricity for their homes and industries.

    However, large cities and industries that are located on rivers often cause problems. As the cities grow in size and industries increase, the water in the rivers becomes polluted with chemicals and other materials. People are learning the importance, however, of doing more to keep their rivers clean if they want to enjoy the benefits of them.

    In nature，you may see many insects and animals of different colors．Have you ever wondered why?

Locusts(蝗虫)are super delicious food for birds，but it is not always an easy job for birds to catch them．It is neither because locusts are good jumpers or runners nor because they are smarter than their enemies．The reason is that the colors of locusts change with the colors of crops(农作物)．When crops are young，locusts look green．But when autumn comes，locusts change to the yellow and brown color as crops do．

    Brown bears，tigers and other animals move quietly through forests．They can't be seen easily by their enemies．This is because they have colors similar to the trees．For the same reason．polar bears that live on a land of snow and ice are white．Butterflies and bees living among the flowers are colorful like flowers．Soil insects are mostly dark—colored and they live under the soil in a dark and wet environment．However，insects with colors different from plants can easily be found and eaten by others．So in order to survive ， they have to hide themselves in the daytime and appear only at night．

    Have you ever noticed some even stranger acts? The ink fish in the sea can send out some very black ink when it faces danger．As the ink spreads over．its enemies will not find themselves suddenly in a dark world．At the same time the ink fish immediately swims away．That is how it keeps itself save though it is not strong at all．

    Do you know kind words and light music can help plants grow better? Many people talk and play music to their plants. And many people try very hard to protect their plants from loud noise, strong light and bad smell. That's not all. Some people are very careful about what they say or do in front of plants. They don't talk about unhappy things and won't beat a fly near their plants.

    Do plants have feelings? Many pupils think so. Even a few scientists think that plants have feelings. But most scientists don't agree. They don't think that plants call be happy, worried, angry, loving or afraid. They don't believe that a plant's feeling can be hurt.

    Professor Cleve Backster in America did an experiment and said that plants have feelings like human beings. He published an article in a magazine in 1968. People began to pay more attention to this topic since then.

    Teenagers are known for being creative and full of new ideas. Let's have a look at these teenage inventions that might change the world.

    When we think of paper, we think of newspapers and books. But there are many other uses. Only half of the world's paper is used for books and newspapers.

    Paper is very good for keeping you warm. You have perhaps seen homeless men sleep on a large number of newspapers. In Finland, it is sometimes-40℃ in winter. The farmers wear paper boots(靴子)in the snow. Nothing could be warmer.

    Each year, more and more things are made of paper. We have had paper cups and plates for a long time. But now we hear that chairs, tables, and even beds can be made of paper. Besides paper boots and shoes, you can wear paper hats, paper dresses, and paper raincoats. When you have used them once, you may throw them away and buy new ones.

    The latest use of paper seems to be paper houses. These are not small houses for children to play in, but real, big houses for people to live in. You can buy a house with three rooms for about 500 dollars. You can put it up by yourself in a few hours, and you can use it for about five years.