D Global warming is the process of earth’s atmosphere heating up. Over the last 100 years, the average temperature of earth’s atmosphere has gone up 1 Fahrenheit. The weather has not changed exactly the same way in every area of the planet. But scientists think that the rise in average temperature is already affecting the earth’s climate. Many scientists now believe that global warming is caused by cutting down trees, producing more trash, and polluting the environment are some of the reasons that the temperature has gone up. Many scientists believe that the biggest causes of global warming are new human technologies that release greenhouse gases into the atmosphere. The greenhouse effect is not new. Certain gases in the atmosphere, like carbon dioxide, nitrous oxide and methane, prevent heat energy from escaping back into space. In the past, the climate didn’t change much because nature produced just the right amount of greenhouse gases to deal with it. Today, most scientists are pretty sure that the rising temperature can’t be blamed on nature. Ever since the industrial revolution in the 1700s, humans have relied on machines for daily life. And many of those machines give off a lot of greenhouse gases. An increase in the release of greenhouse gases from human activities is throwing nature off balance. The climate is a very complicated thing, but many scientists agree that the rising atmospheric temperature has already damaged the environment. Sheets of ice, called glaciers, are melting in Antarctica and other parts of the globe. As glaciers break off and melt into the oceans, they are adding warm water to the oceans and causing the sea level to rise. Over the past last 100years, the sea level has risen 6-8 inches around the world. That means land along the coasts is beginning to disappear under water. Bigger and warmer oceans are also adding to other weather problems caused by pollution in the atmosphere. Some places have received more rain, others have had bigger storms and a few areas in the world have experienced unusual droughts. 68. What is mainly talked about in this passage? A. The melting of glaciers.                B. Global warming. C. The world’s weather.                  D. The earth’s temperature. 69. What causes global warming according to scientists? A. Human activities.                     B. The nature itself. C. The earth’s atmosphere.                D. New discoveries 70. How can greenhouse gases make the globe warm? A. They keep heat in the atmosphere B. They let the heat go out into space. C. They release heat into the air. D. They can make the other gases warm. 71. From the passage we can see that global warming will bring about _________. A. the pollution in the atmosphere            B. natural disasters C. population pollution                     D. the rise of glaciers