4. We can conclude from the passage that __________. A. while pushing asteroids off course nuclear weapons would destroy the world. B. asteroids racing across the night sky are likely to hit Earth in the near future. C. The worry about asteroids can be left to future generations since it is unlikely to happen in our lifetime. D. Workable solutions still have to be found to prevent a collision of asteroids with Earth.

3. What do people think of the suggestion of using nuclear weapons to change the course of asteroids? A. It sounds practical but it may not solve the problem. B. It may create more problems than it might solve. C. It is a waste of money because a collision of asteroids with Earth is very unlikely. D. Further research should be done before it is proved practical.

2. What do scientists say about the collision of an asteroid with Earth? A. It is very unlikely but the danger exists. B. Such a collision might occur once every 25 years. C. Collisions of smaller asteroids with Earth occur more often than expected. D. It's still too early to say whether such a collision might occur.

1. What does the passage say about asteroids and meteoroids? A. They are heavenly bodies different in composition. B. They are heavenly bodies similar in nature. C. There are more asteroids than meteoroids. D. Asteroids are more mysterious than meteoroids.

4. Suppose the earth's temperature rose by 3 degrees __________. A. great harm would be done to mankind. B. the sea level would go up by 10 centimeters. C. all the land in world would be flooded. D. crops would be unable to grow on earth.

3. Which of the following is not true? A. Burning too much coal and oil produces lots of greenhouse gases. B. It has become warmer on the earth now than in the past. C. Gases put into the atmosphere now will effect the earth years later. D. The temperature in a greenhouse is as high as that in the atmosphere.

2. If there were no greenhouse effect, __________. A. no plants would grow on our planet. B. the earth would be 30 degrees colder. C. man could not live on the earth. D. all of the above answers are correct.

1. "Greenhouse effect" means __________. A. the way in which gases are used for keeping plants warm. B. a natural course which is to some degree helpful to us. C. the way 'greenhouse gases' trap heat on the earth. D. the whole course in which greenhouse gases prevent heat.