10．Coral reefs
For millions of years，coral reefs have provided homes and food for thousands of different living things．Fish and sea birds share the reef with other sea creatures．Now these beautiful places are in danger．（36）CScientists have found that people and pollution have ruined more than one-fourth of the earth's coral reefs．Unless things change，all of the remaining reefs may die within your lifetime．
Some people think that coral is stone because of its rough，hard surface．But coral is an animal!Tiny polyps （水螅体） form the coral reefs．（37）AThese colours come from the algae （海藻） living inside the coral．Billions of coral polyps stick together．New ones grow on the dead corals．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．（38）FThe coral reefs have been harmed in different ways．People have broken off pieces of coral．They wanted to sell or keep them．To catch more fish，people have dropped sticks of dynamite （炸药） into the water．（39）EWater pollution has encouraged overgrowth of the sea plants that grow near coral reefs．They block out the sunlight that the algae need．（40）DWarm water kills the algae．When the algae die，the coral loses both its food and its color．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 colours．
B．This has blown up parts of the reefs．
C．So are all the sea plants and animals that depend on them．
D．The worst problem is the heating up of the world's oceans．
E．Coral reefs are experiencing a rapid increase in most locations．
F．However，it has taken human beings less than 100 years to destroy the reefs．
G．They cover less than 1% of the ocean floor，but support about 25% of sea creatures．
