Nine planets move around the sun, but the earth has the best (36) position among the planets. Because it doesn’t (37) travel too near or too far from the sun, and it receives the necessary (38) amount of heat and light. The air around the earth keeps out the sun’s dangerous (39) rays. The sun dries up some of the (40) ocean water, but the air above turns it into rain. Sun, air, and water give us the change of seasons from (41) autumn and winter to spring and summer, when the earth becomes green with (42) vegetation. Without the earth’s special (43) environment, there would be no life on earth. (44) If we change this environment, we may destroy some of the life on earth.

　　The earth is 4.5 billion years old. At first it was a dead object of rock, without air or water, now the earth is home to a variety of living things. (45)All the varieties of plants and animals are dependent on the sun, on air and water, and on each other. The sun will shine for millions of years into the future. But will we be able to keep the earth a special place among the planets? The future of the earth doesn’t look bright. By the year 2000 there will be over six billion people on our planet. In twenty years bad air will produce more serious health problems. (46) If we cut down too many trees, many birds and wild animals won’t have a home. We will destroy our environment if we don’t plan for the future.