Read the passage and choose the correct answers.

Greenhouse gases refer to the atmosphere gases that cause greenhouse effect. Some of these gases happen naturally, while others are caused by human ac- tivities. Carbon dioxide, methane, ozone, nitrous oxide, and water vapor are some examples of greenhouse gases.

Greenhouse gases act like a natural blanket to the earth. They keep the temperature of the earth warmer and longer. Greenhouse gases cause many environmental and ecosystem problems.

Because the temperature of the globe is warmer, the glaciers and polar ice caps begin to melt. As a result, sea levels have risen. The studies also show that

floods and droughts are more severe. There are also more reports of extreme weather, like hurricane Katrina. There will be more tidal wave and beach erosion on the coasts.

Many species of plants and animals also face the danger of extinction from global warming, because they will not be able to adapt to the change in the weather and climate. For instance, the rises in sea level can increase the salinity of freshwater across the globe. Many species of freshwater fish and plants will be affected by this change. It is estimated that over a million species of plants and animals will be extinct by 2050.

The third paragraph is about_______