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1、Level 3 unit 3 part1 Listening There are many forms of life on Earth, including human beings. Life exists in a variety of conditions. Some forms of life live in a watery environment, like the oceans. Other forms of life can be found in very dry areas, like deserts. However, for any form of life to e
2、xist, conditions must be right. When conditions are not right, that form of life will become extinct. To become extinct means to die out completely. -What must be right for life to exist? -Conditions. -What happens if conditions are not right? Life cannot exist. Conditions must be right for life to
3、exist. Millions of years ago, there were forms of life that no longer exist. When conditions changed, these forms of life died out. One extinction event happened about 250 million years ago. This was the largest extinction event of all time. Many forms of life became extinct. 96% of all life in the
4、oceans died out. Most insects also became extinct. This event happened over a period of several million years. -What happened about 250 million years ago? Many forms of life became extinct. -What happens in an extinction event? Many forms of life became extinct. The causes of this extinction event a
5、re still unknown. Possible causes include large volcanic eruptions and global warming. Some scientists believe that there were several causes. They believe that a series of events caused the extinctions. Scientists are working to better understand what really happened. -What is the one possible caus
6、e of this event? Volcanic eruptions. In modern times, we humans face changing conditions. For humans to live, we need clean air and clean water. Pollution is now a growing problem around the world. Pollution poisons the air and water that we depend on. Polluted air makes people sick and afraid to go
7、 outside. Polluted water poisons our food supply. As a result, we never know which foods are safe to eat. Human beings cannot live in a poisoned environment. Therefore, pollution is a major threat to our existence. -What do humans need to live? Clean air and clean water. -What poisons the air and wa
8、ter? Pollution. Humans need temperatures to be in a comfortable range. To be in a comfortable range means to be neither too hot nor too cold. With global warming, global temperatures are rising. As temperatures rise, the polar icecaps will melt. As the polar ice caps melt, ocean levels will rise. Ar
9、eas of some countries will soon be under water. People will be forced to relocate from flooded areas. -As temperatures rise, what will happen to the polar icecaps? The polar icecaps will melt. -As the polar icecaps melt, what will happen? Ocean levels will rise. In nature, even small changes can som
10、etimes have large effects. Its difficult to predict whats going to happen. The entire ecosystem that we depend on is changing. Some of these changes are irreversible. Irreversible changes cannot be undone. -What can small changes sometimes cause? Large effects. -What is happening to the worlds ecosy
11、stem? Ecosystem is changing. Lets hope that humans are smart enough to understand how the world is changing. With more understanding, we can make better choices about what to do. We can face the challenges of pollution and global warming. We need to do this before its too late. This planet Earth is
12、our only home and we need to protect it. -What challenges do we need to face? Pollution and global warming. We need to protect the environment that supports us. Vocabulary Mammals are covered by hair or fur, have a backbone, and are warm blooded. All female mammals produce milk for their young. Rept
13、iles are covered by scales, and includes snakes, lizards and turtles. Reptiles have a backbone and are cold blooded, which means they often relied on the external source of heat. Birds are covered by feathers and are warm blooded. Most birds can fly and many types of birds migrate great distances. M
14、ost insects such as ants and bees have a small three part of body with three pairs of legs. Some insects, like mosquitoes, spread diseases that cause the deaths of many humans. Unlike animals, plants get the energy that they need from the sun. Plants convert light energy, along with carbon dioxide a
15、nd water, into chemical energy. Insects dont have a backbone and most are cooled-blooded. Insects such as ants and bees live in well-organized colonies. Mosquitoes cause the deaths of more humans than any other animal. Mammals include some of the most intelligent animals on earth, such as elephants
16、and human beings. Scales and rulers are used to measure weight and length. Units of weight include kilograms and pounds, and units of length include centimeters and inches. These instruments are used to observe very large and very small objects. Telescopes are used to by astronomers and microscopes are used by biologists and doctors. These appliances are used in the kitchens of almost every home. Stoves are used to heat food and refrigerators are used to keep foods cool or cold
