A、Space Travel. B、Problems of Astronauts. C、Water Pollution. D、The Spaceship Earth. D

Spaceships travel around the earth, go to the moon, and return home safely. The 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 sup- plies, just like the limited supplies on the astronauts’ spaceship, have to be used carefully because we can’t 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 tile 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 Earth’s resources wisely; without destroying them. To save the spaceship Earth, we must cooperate with nature and learn better ways to use, not abuse, our environment.