听力原文： Spaceships travel around the earth, go to the moon, and return home safely. T

he astronauts carry important supplies with them on the spaceship — food, water and air. Sometimes there are problems on the spaceship, and the astronauts have to understand how the spaceship works in order to repair the problems. In a way, all of us are really on a spaceship, the planet Earth. We move around the Sun at 18 miles per second and never stop. On our spaceship we have four billion people and a limited supply of air, water and land. These supplies, just like the limited supplies on the astronauts spaceship, have to be used carefully because we cant buy new air, water or land from anywhere else. Everyone needs air, water and land to live — this is our environment. The environment on our planet is a closed system; nothing new is ever added. Nature recycles its resources. Water, for example, evaporates and rises to form. clouds. This same water returns to the earth as rain or snow. The rain that falls today is actually the same water that fell on the dinosaurs 70 million years ago. Over the years, people have changed the environment. We have poured back into the land, air, and water more wastes than nature can clean. So we have pollution today. To continue to survive, we must learn how to use the Earths resources wisely, without destroying them. To save the spaceship Earth, we must cooperate with nature and learn better ways to use, not to abuse our environment. Questions 20 to 22 are based on the passage you have just heard. 20. Which would be the best title for the passage？ 21. Which is true according to the passage？ 22. What can we conclude from the passage？20．

A．The present population on earth is fourteen billion.

B．Water is recycled every seventy million years.

C．The planet earth travels around the sun at 18 miles per second.

D．The earth is compared to a spaceship because they both travel around the sun.