题目内容：

    They may be small, but they build big things! Coral(珊瑚虫), which live in the warm parts of the Earth's oceans, are probably the biggest builders on the planet. Coral polyps turn calcium(钙)from seawater into a kind of hard stone. Slowly, they build up a hard skeleton(外骨骼)around their bodies. When polyps die, their skeletons remain. Young polys attach themselves to the old skeletons and make new skeletons. Over time, various shapes are slowly built up into amazing coral reefs(珊瑚礁).

Scientists sometimes think of coral reefs as underwater cities. There are nearly a thousand coral species. Reefs are also home to millions of sea creatures, like fish, crabs, turtles, and sharks.

Humans don't live in coral reef cities, but we get a lot from them. Reefs create jobs for people in the fishing industry and other related businesses. They also supply us with food. Reefs protect our coasts-the coral slows waves down and protects beaches from wearing. Chemicals from reef creatures help scientists create new medicines, which help doctors treat different illnesses.

Coral reefs are very important, yet we don’t take good care of them. Environmental problems have already killed about twenty percent of the world’s reefs. About half of the remaining reefs are dying, and experts believe all of Earth's coral reefs will be in danger by 2050.

People catch too many reef fish and often damage the reefs. Polluted water also causes problems because reef-destroying algae(海藻)grow in dirty water. Even air pollution hurts coral reefs. Global warming causes warmer ocean water, which can cause polyps to lose helpful algae. Without the algae, coral turns white. If it continues, the coral dies.

Can we save coral reefs? Experts say yes-if we make hard choices. Many people around the world are working to protect reefs. Humans and coral polyps are very different, but we both build amazing cities. We will both win if people protect our beautiful oceans.

1.What can be learned about coral reefs?

A.They turn calcium into stones. B.They can be found in cold waters.

C.They provide food for coral polyps. D.They are formed by the growth of coral

2.How do coral reefs help humans?

A.By taking in pollution. B.By protecting beaches.

C.By improving the environment. D.By giving out important chemicals.

3.The helpful algae give coral its         .

A.hardness B.size C.color D.shape

4.What would be the best title for the text?

A.Cities in the sea B.Reefs in trouble

C.Why are reefs important? D.How can we protect coral reefs?