A、Because the Earth is heavily polluted. B、Because nature cannot recycle its resources. C、Because there are more and more people

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 five billion people and limited supply of air, water, and land. These supplies have to be used careful because we can’t buy new air, water, or land from anywhere else.
    The 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. 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 land 70 million years ago.
    Today, the Earth is in trouble. Factories pour dirty water into our rivers. Many fish die and the 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 the roadside becomes covered with al sorts of wastes. Over the years, people have changed the environment, and we have pollution.
    To continue to survive, we must learn how to use the Earth’s resources wisely. We have to change our habits and stop dumping 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.