You may think there is only sand in the desert of the world, but it is not true. In the desert, as we know, there is a little rain, but it is not enough for most plants. Still we can see some plants live in the desert.

There is water in some places in the desert. We call these places oases(绿洲). In the oases, there are villages and towns. People grow all kinds of crops in the fields there.

People also live outside the oases. They have camels, sheep and other animals. These animals depend on the desert plants for their food and do not need too much water.

The animals are useful to the desert people in many ways. They eat the meat and drink the milk of the animals. They use the camels for carrying water, food, tents and something else. 

The people of the desert have to keep moving from place to place. They must always look for grass or desert plants for their animals. When there is no more food for their animals, they move to another place. The desert people are friendly. No man in the desert would ever refuse to help the people in trouble and give them food and water.

The earth is our home; we must take care of it. This means keeping the land, air and water clean.

Pollution is a "dirty word". To pollute means to make things unfit (不合适的) or unclean to use.     34     We see it, smell it, taste it and drink 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 a fire, washed his clothes in the river and threw his waste on the ground.     35     At first the problem was not so serious because there was plenty of clean air, land and water.     36     All the dirty things could be absorbed (吸收) by nature and soon be covered over. But this is no longer true. The increase of the population and the development of industry have changed that. Man is slowly polluting his environment. 

Through the use of poison (毒药), man has polluted land and killed the animals. By putting dirty water and chemicals into rivers and lakes, we have polluted our drinking water, killing the fish.     37     More people, more waste. 

Where is this all to end? Are we turning the world into a big rubbish dump, or is there any hope that we can get rid of pollution?     38     A large number of people are now working hard to bring pollution under control.

Have you ever heard of friendships among animals? At a zoo in Germany, an unusual friendship developed     39     a cat and a bear. 

No one is quite sure how the friendship began. One day, some     40     in the zoo noticed a black cat was in the bear's cage(笼子), and the two animals were playing together peacefully. They were surprised     41     well the small cat seemed to get along with the large bear. The bear keeper allowed the cat to stay with the bear. And later the pair and their unusual friendship     42     a hot topic at the zoo. 

After several     43    , a new cage was built for the bear. However, the cat was still in the old cage, so it walked around     44     and cried to be with the bear. The bear keeper finally took pity on the cat     45     let it stay with the bear. The cat and the bear got     46     than before. They often sat together and shared meals in     47     sun. 

Sometimes it may be natural for animals     48     their own ideas about who would make a good friend. These may surprise humans, who usually expect animals to form relationships with others of the same species.