题目内容：

For millions of years, coral reefs（珊瑚礁）have provided homes and food for thousands of different living things. Fish and sea birds share the reef with other creatures. Now these beautiful places are in danger.    1.   Scientists have found that more than one-fourth of the earth’s coral reefs have been ruined so far. Another quarter will disappear in ten years’ time. Unless things change, all of the remaining reefs may die within your lifetime.

Some people think that coral is stone because of its rough（粗糙的）, hard surface. But coral is an animal. Tiny polyps（水螅体）form the coral reefs.    2.  These colors come from the algae（海藻）living inside the coral. Billions of coral polyps stick together. New ones grow on the dead ones. This happens year after year. Over time, the coral builds up a reef. The reef rises from the ocean floor until it almost reaches the sea’s surface. It takes coral 500,000 years to build a huge reef.   3.

The coral reefs have been harmed mainly in three ways. The first factor is pollution on land. The pollutants run with rainwater into streams and rivers, which carry the poisons into the ocean. 4.   So they will be easily infected with diseases. Another cause is people. People have broken off pieces of coral, which they want to sell or keep as souvenirs. People have dropped sticks of dynamite（炸药）into the water to catch more fish. The worst problem is the heating up of the oceans. Higher ocean temperatures kill the important food source for the coral—the algae, the tiny greenish-gold water plants that live on coral.   5.  The coral turns white and dies. Scientists call this coral bleaching（珊瑚白化）. The bleached part of the coral reef cannot recover.

A. They come in many different colors.

B. However, humans have taken less than 100 years to destroy it.

C. This has blown up parts of the reefs.

D. When the algae die, the coral loses both its food and its color.

E. So are all the sea plants and animals that depend on them.

F. Chemicals from the poisons kill reefs or make them weak.

G. Many of them might not even last that long!