Coral reefs(珊瑚礁)are not just beautiful to look at, but also very useful. They shelter nearly 25% of ocean life and provide food for billions of people. However, like many other species，this amazing ecosystem(生态系统)is in danger of dying out!
There are two main reasons for the destruction—the rising carbon dioxide levels caused by global warming as well as human influences like bottom operation．While not much can be done about the former, the loss caused by the latter can be reduced.
Bottom operation involves dropping nets to the ocean bed and then dragging them up when filled with fish. This action damaged the ocean floor, causing the coral branches to separate from the reefs. While that is very discouraging, the good news is that if these branches are found quickly, they can be saved. At present, that work is being performed by volunteer divers. But the method has only limited success because they cannot stay long underwater, nor reach depths of over 200 meters where some of the deep-sea coral grows.
However, this is not the case for Coralbots—the robots that can constantly seek out broken branches. A team of ‘coralbots’, each individually working to simple rules will piece together damaged bits of coral, allowing them to regrow. They are programmed to be able to distinguish between other things and the coral. The robots work together in groups, similar to bees. If one coralbot is damaged then the others will still be able to complete the task.
Dr Lea-Anne Henry said: “The project of using coralbots offers us the possibility to restore(修复)the function of reefs．It is in the final stage of testing. Once ready, the researchers hope to build hundreds of them and use them effectively in the waters off Scotland and then hopefully in the oceans across the world.”
21. Which of the following is a cause of the danger that coral reefs are facing?
A. The appearance of new creatures
B. The seriously polluted sea water
C. The activities of volunteer divers
D. Human activities like bottom operation
22. Compared with volunteer divers, coralbots ______________．
A. can stay longer underwater B. must work as bees do
C. can distinguish what is coral D. can never be damaged
23. What does the underlined word “them” in the last paragraph refer to?
A. Coralbots B. Broken branches
C. Working bees D. Bits of coral
24. We can infer from the passage that _______________．
A. bottom operation has affected most underwater creatures
B. broken branches of coral can regrow if saved in time
C. hundreds of the coralbots have worked well in the oceans
D. with the coralbots all damaged coral reefs can be restored