Questions 6 to 10 are based on the following passage: 

Half of the world's coral reefs （珊瑚礁）have died in the last 30 years. Nowscientists are racing to ensure that the rest survive. Even if global warning wereto right now, scientists predict that more than 90% of corals will die by Ifno major steps are taken to address the problem, the reefs may be headed for totalextinction （灭绝）. 

(78) The planet ’s health depends on the survival of coral reefs. Theydescribed as “the rainforests of the sea", because they provide shelter for a widevariety of sea life. In addition, the reefs serve as barriers that protect coastlinesfrom the full force of powerful storms.Coral are used in medical research for cures to diseases. They are key to localeconomies as well, since the reefs attract tourists, the fishing industry, and otherbusinesses, bringing in billions of dolllars. 

（79）Corals are particular sensitive to changes in temperature. A rise ofjust 1 to 2 degrees can force the corals to drive out the algae （水藻）. Then thecorals turn white in a process called “bleaching”. Corals can recover fromshort-term bleaching, but long-term bleaching can cause permanent damage. In 1998,when sea surface temperatures were the highest in recorded history, coral reefsaround the world suffered the most severe bleaching. It is estimated that even underthe best of conditions, many of these coral reefs will need decades to recover.