In a way, all of us are on a spaceship -the planet Earth. We move around the sun at 18 miles per second and never stop. On our spaceship, we have 5 billion people and limited supply of air, water and land. These supplies have to be used carefully because we can't buy new air, water or land from anywhere else. Environment on our planet is a closed system. Nothing new is ever added. Nature recycles its resources. Water, for example, evaporates and rises as visible drops to form clouds. The same water returns to the earth as rain or snow. The rain that falls today is actually the same water that fell on the land 70 million years ago. Today, the earth is in trouble. Factories pour dirty water into our rivers, many fish die and water becomes unhealthy for people to drink. Cars and factories put poisons into the air and cause plants, animals and people to get sick. People throw bottles and paper out of their car windows and roadside becomes covered with all sorts of wastes. Over the years people have changed environment and we have pollution. To continue to survive, we must learn how to use the earth resources wisely. We have to change our habits and stop dropping such enormous amounts of industrial waste into the water and air. We must cooperate with nature and learn better ways to use, not abuse our environment.

Questions 17 to 20 are based on the passage you have just heard.

17. Why does the speaker compare the earth to a spaceship?

18. How fast does the planet Earth travel around the sun?

19. Why does the speaker advise us to use the earth resources wisely?

20. What message is the speaker trying to convey?