A．that

B．where

C．when

D．which

After the “Big Bang(宇宙大爆炸)”,the earth was just a cloud of energetic dust. What it was to become was a mystery until the dust began to slowly combine into a ball moving around the sun. The problem was that the earth became violent because it was not clear whether the solid shape was to last or not. It exploded loudly with fire and rock, which were in time to produce the water vapor, carbon dioxide, oxygen, nitrogen and other gases, which were to make the earth’s atmosphere. As the earth cooled down, water began to appear on its surface. Nobody knew that it was going to be different from other planets going round the sun. Water had already disappeared from planets like Mars or satellites like the moon, but it stayed on the earth. This is how the earth began to show its special qualities. Many people think that this was important for the beginning of life. It allowed the earth to dissolve harmful gases, which had become part of the earth’s atmosphere, into the oceans and seas. That made it possible for life to begin to develop.

What scientists think is that the earth was different because of the arrival of small plants growing in the water on its surface. Nobody understood that these plants were the start of many changes. They multiplied and filled the first oceans and seas with oxygen .This encouraged the development of early shellfish and all sorts of fish.

Many millions of years later the first green plants began to appear on land. Scientists believe that taking carbon dioxide from the atmosphere and filling the air with oxygen helped life to develop. Later land animals appeared. Some were insects. Others, called amphibians, were able to live on land as well as in the sea. When the plants grew into forests, reptiles appeared for the first time. They produced young generally by laying eggs. Later, some huge animals, called dinosaurs, developed, They laid eggs too. They became the most important animals on the earth for millions of years. But when they disappeared, mammals became more important. They were the last group of animals and they were different because they produced their young from within their bodies.

Small clever animals, now with hands and feet, appeared and spread all over the earth. They developed new methods of growing food, hunting and moving around. As time went by they covered the earth and have become the most important animals on the planet. But they are not taking care of the earth very well. They are putting too much carbon dioxide into the atmosphere, which prevents heat from escaping from the earth into space. The earth may become too hot for the lives on it. Whether life will continue on the earth for millions of years to come will depend on whether this problem can be solved.

1. Why was the earth different from other planets?

A. It produced a lot of heat. B. The water remained.

C. Water disappeared.　　　 D. It was the oldest planet.

2. Why was life able to develop on the earth but not on other planets?

A. The earth had a solid shape.

B. The earth did not have harmful gases in its atmosphere.

C. The water stayed on the earth but not on other planets.

D. The earth was not too hot and not too cold.

3. Why was it necessary for plants to grow before animals?

A. Animals needed plants to protect them from the sun.

B. Plants provided oxygen for animals to breathe.

C. Animals could hide from hunters in forests.

D. It was easier for plants to grow.

4. What problem is caused by human beings?

A. They exist everywhere on the earth.

B. They cause global warming.

C. They find new methods of growing crops.

D. They enjoy hunting and fishing.