1 . Nearly 40 years ago, Peter Harrison, a marine ecologist witnessed the first recorded large-scale coral bleaching（珊瑚白化）event. Diving in the Great Barrier Reef（大堡礁）, he was shocked by the scene before him. "The reef was made up of healthy corals and badly bleached white corals, like the beginnings of a ghost city," he says. Just months before, the same site was filled with colorful tropical life.
"Many of the hundreds of corals that I'd carefully tagged and monitored finally died," he says. "It was shocking and made me aware of just how weak these corals really are.”
Coral exists together with photosynthetic algae（藻类）, which live in its tissues and provide essential nutrition（and coloration）. But high temperatures and other stresses can turn algae poisonous. When this occurs, the algae may die or be removed by the coral, a process known as bleaching because the coral's clear tissue and white calcium carbonate skeleton（碳酸钙骨骼）are exposed. If the coral can't reestablish its link with algae, it will starve or become ill.
The widespread destruction Harrison saw in 1982 was repeated on many other Pacific Ocean reefs that year and the next. In 1997 and 1998 the phenomenon went global, killing some 16 percent of the world's corals. With rising temperatures, pollution, disease, increased ocean acidity, invasive species, and other dangers, Harrison's ghost cities are expanding
Scientists suppose that about four decades ago severe bleaching occurred roughly every 25 years, giving corals time to recover. But bleaching events are coming faster now—about every six years—and in some places soon they could begin to happen annually.
"The absolute key is dealing with global warming," says marine biologist Terry Hughes. "No matter how much we clean up the water, the reefs will die." In 2016, a record-hot year in a string of them, 91 percent of the reefs that consist of the Great Barrier Reef bleached.
1. Peter Harrison was shocked when diving in the Great Barrier Reef, because___________.| A．the reefs were made up of precious corals | B．the corals were ruined badly and quickly |
| C．he found a ghost city with tropical life | D．he saw the corals he had tagged before |
| A．the causes of coral bleaching | B．the weakness of corals and algae |
| C．the elements that make algae die | D．the process of building a link with algae |
| A．global warming | B．the polluted ocean | C．the white corals | D．invasive species |
| A．With algae living in its tissues, coral's white skeleton is exposed. |
| B．Solving global warming is the real solution to coral bleaching. |
| C．The reefs die because the water hasn't been cleaned thoroughly |
| D．The severest coral bleaching occurred about four decades ago. |
