托福阅读真题Official 47 Passage 3（六）

托福阅读真题Official 47 Passage 3（六）

Coral Reefs

An important environment that is more or less totally restricted to the intertropical zone is the coral reef.Coral reefs are found where the ocean water temperature is not less than 21°C,where there is a firm substratum,and where the seawater is not rendered too dark by excessive amounts of river-borne sediment.They will not grow in very deep water,so a platform within 30 to 40 meters of the surface is a necessary prerequisite for their development.Their physical structure is dominated by the skeletons of corals,which are carnivorous animals living off zooplankton.However,in addition to corals there are enormous quantities of algae,some calcareous,which help to build the reefs.The size of reefs is variable.Some atolls are very large—Kwajalein in the Marshall Islands of the South Pacific is 120 kilometers long and as much as 24 kilometers across—but most are very much smaller,and rise only a few meters above the water.The 2,000 kilometer complex of reefs known as the Great Barrier Reef,which forms a gigantic natural breakwater off the northeast coast of Australia,is by far the greatest coral structure on Earth.

Coral reefs have fascinated scientists for almost 200 years,and some of the most pertinent observations of them were made in the 1830s by Charles Darwin on the voyage of the Beagle.He recognized that there were three major kinds:fringing reefs,barrier reefs,and atolls;and he saw that they were related to each other in a logical and gradational sequence.A fringing reef is one that lies close to the shore of some continent or island.Its surface forms an uneven and rather rough platform around the coast,about the level of low water,and its outer edge slopes downwards into the sea.Between the fringing reef and the land there is sometimes a small channel or lagoon.When the lagoon is wide and deep and the reef lies at some distance from the shore and rises from deep water it is called a barrier reef.An atoll is a reef in the form of a ring or horseshoe with a lagoon in the center.

Darwin’s theory was that the succession from one coral reef type to another could be achieved by the upward growth of coral from a sinking platform,and that there would be a progression from a fringing reef,through the barrier reef stage until,with the disappearance through subsidence(sinking)of the central island,only a reef-enclosed lagoon or atoll would survive.A long time after Darwin put forward this theory,some deep boreholes were drilled in the Pacific atolls in the 1950s.The drill holes passed through more than a thousand meters of coral before reaching the rock substratum of the ocean floor,and indicated that the coral had been growing upward for tens of millions of years as Earth’s crust subsided at a rate of between 15 and 51 meters per million years.Darwin’s theory was therefore proved basically correct.There are some submarine islands called guyots and seamounts,in which subsidence associated with sea-floor spreading has been too speedy for coral growth to keep up.

Like mangrove swamps,coral reefs are extremely important habitats.Their diversity of coral genera is greatest in the warm waters of the Indian Ocean and the western Pacific.Indeed,they have been called the marine version of the tropical rain forest,rivaling their terrestrial counterparts in both richness of species and biological productivity.They also have significance because they provide coastal protection,opportunities for recreation,and are potential sources of substances like medicinal drugs.At present they are coming under a variety of threats,of which two of the most important are dredging and the effects of increased siltation brought about by accelerated erosion from neighboring land areas.

Question 11 of 14

According to paragraph 4,why have coral reefs been compared to tropical rain forests?

A.Both are located near the Indian and western Pacific Oceans.

B.Both are home to a wide variety of species.

C.Both play an important role in protecting coastal lands.

D.Both are threatened by the erosion of nearby land.

正确答案：B

题目详解

题型分类：修辞目的题

题干分析：问及段落间关系的题目阅读每段主旨和结构，特别注意上段结尾下段开头

选项分析：

通过coral reefs和tropical rain forests定位第三句Indeed,they have been called the marine version of the tropical rain forest,rivaling their terrestrial counterparts in both richness of species and biological productivity说明coral reefs和tropical rain forests一样，物种丰富，因此选项B正确。

A选项，对应本段前半句，但是作为一个介词短语，不是重点信息，对比两者主要为了说明他们的物种都很丰富，与原文矛盾。

C选项，对应原文倒数第二句，保护海岸岛屿，但与比较两者的目的无关，无关选项。

D选项对应原文最后一句，只是说了珊瑚受到的威胁之一，与比较目的无关。

Question 12 of 14

The word“potential”in the passage is closest in meaning to

A.powerful

B.possible

C.valuable

D.reliable

正确答案：B

题目详解

题型分类：词汇题

选项分析：

原文说珊瑚也具有重大意义，因为他们提供保护海岸，娱乐机会以及药物的potential sources，推测这些益处都是比较潜在的，但是目前人类还没有开发他们，因此用选项B替换更符合语境，

A选项，有力量的。

C选项，有价值的。

D选项，可靠的。

带入原文均不符合语境。