Read the following passage and mark the letter A, B, Cor D on your answer sheet to indicate the correct word or phrase that best fits each of the number blanks.

For many years, scientists have been arguing about global warming. While they don't all agree about what this might mean for Planet Earth, many think it is going on — and that we can do something about it. We need a few more years to gather facts before anyone can say for sure. But people need to start now to do things to stop it.

There are many types of gases in our atmosphere. Some of them absorb the heat from the sun and control our climate. "Greenhouse gases” are those that hold the heat around the planet's surface. When we increase the level of these gases, we upset the balance. These extra gases trap even more heat.

Some scientists say global warming has already started to change the environment. The Earth's temperature has risen one degree Fahrenheit this century. Many scientists believe it could rise from two to eight degrees Fahrenheit in the next hundred years. This would be the fastest rise in 10,000 years.

Sea levels could rise. Ice sheets could melt and add to the water level. Many places near the coast would be in danger of flooding. If the climate change is too large and too fast, some plants and animals could really suffer. They could even die out. Weather could become more violent and extreme. In some places there would be more forest fires. But in other areas there would be more rain and snow, storms and floods.

The word "it" in paragraph 3 refers to ______.