懂你英语Lv3-Unit-3.docVIP

Unit 3-1 Listening There are many forms of lives on earth, including human beings. Life exists in a variety of conditions. Some forms of live live in the watery environment, like the oceans. Other forms of lives can be found in very dry areas like desserts. However, for any form of live to exist, conditions must be right. When the conditions aren’t right, that form of lives can extinct. To become extinct means to die out completely. Conditions must be right for life to exist. Millions of years ago, there were forms of lives that no longer exist. When condition changed, this form of life died out. One extinction event happened about 250 million years ago. This is the largest extinction event of all time. Many forms of lives become extinct. 96% of all lives in the oceans died out. Most insects also became extinct. This event happened over a period of several million years. The causes of this extinction event are still unknown. Possible causes include largest 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 for better understand what really happened. In modern times, we human beings 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 get outside. Polluted water poisons our food supply. As a result, we never know which foods are safe to eat. Human beings can be not live in a poisoned environment. Therefore, pollution is a major threat to our existence. Humans need temperature 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 global temperatures rise, the polar icecaps will melt. As the polar icecaps melt, ocean levels will rise. Areas of some countries wil