根据短文内容，从下边的A—F选项中选出能概括每一段主题的最佳选项。选项中有一项为多余选项。A . The different structure of blue holes.B.

根据短文内容，从下边的A—F选项中选出能概括每一段主题的最佳选项。选项中有一项为多余选项。
A . The different structure of blue holes.
B. Why scientists are interested in blue holes.
C. How a blue hole comes into being.
D. Blue holes exploring is becoming popular.
E. Rules to obey when diving into blue holes.
F. The general definition of a blue hole.
【小题1】
A blue hole is a flooded sea cave with a hole that opens up at the land's surface. These cave systems form in carbonate rock, often on islands. Some blue holes have very special rock formations and water chemistry. Far below sea level, they contain some of the harshest environments on Earth, with no oxygen and no light. Yet these areas are filled with life forms that have adapted to the extreme conditions.
【小题2】
Blue holes get their name from the color some have when seen from the air. The color is usually a reflection of the sky on the water. Blue holes are the result of erosion in which water breaks down rock. Rain falling thousands of years ago contained chemicals which slowly wore away at the limestone landmass. These holes later filled with sea water as the sea level changed. The rising and falling of sea levels and the mixture of salt and fresh water further wore away at these cave formations.
【小题3】
These flooded cave systems can be found in the ocean, or they can be found inland. Ocean caves are affected by tides, so they always have water movement. But blue holes on land are very still. They have several layers of water, chemicals, and bacteria. The top layer of fresh water comes from rainfall. This layer acts like a cap on top of the layered mixture, and keeps out oxygen from the atmosphere. The fresh water floats on a denser layer of saltwater. Underneath this is a layer of poisonous hydrogen sulfide, produced by bacteria living in the water. Underneath this layer is anoxic seawater -- water that does not contain any oxygen.
【小题4】
Information gathered from these blue holes is helping scientists to increase their understanding of biology, archaeology and geology. But exploring these blue hole environments brings danger as well as discovery. Scientists are interested in these caves because oxygen-free conditions there are similar to those on Earth long ago, before oxygen existed on our planet. But there are many difficulties involved in diving into blue holes.
【小题5】
One reason blue holes have not been fully explored is that they can be extremely dangerous. There are many safety rules that divers must follow to help ensure their survival. First, divers must have training and experience to swim in these caves. Divers who explore a cave for the first time must establish a thin rope called a guideline. This line helps them to safely enter and exit the cave without getting lost.F

(答案→)F 解析：本文主要讲述的是blue hole如何形成，具体结构以及科学家对它产生兴趣的原因，最后还提及了人们在研究它的时候所要遵循的基本规则。【小题1】F 根据本段第一句A blue hole is a flooded sea cave with a hole that opens up at the land's surface.可知本段主要介绍的是什么是blue hole，也就是blue hole的定义。故F正确。【小题2】C 根据本段2,3,4行Blue holes are the result of erosion in which water breaks down rock. Rain falling thousands of years ago contained chemicals which slowly wore away at the limestone landmass. These holes later filled with sea water as the sea level changed.可知本段主要讲述的是blue hole是如何形成的，也就是分析它产生的原因。故Ｃ项正确。【小题3】A 根据本段2,3,4,5行They have several layers of water, chemicals, and bacteria. The top layer of fresh water comes from rainfall. This layer acts like a cap on top of the layered mixture, and keeps out oxygen from the atmosphere. The fresh water floats on a denser layer of saltwater. Underneath this is a layer of poisonous hydrogen sulfide, produced by bacteria living in the water.可知主要讲述的blue hole的构造和不同的结构中的具体的组成。故A项正确。【小题4】B 根据本段3,4行Scientists are interested in these caves because oxygen-free conditions there are similar to those on Earth long ago, before oxygen existed on our planet.可知本段主要解释科学家对于blue hole感兴趣的原因。故B正确。【小题5】E 根据本段第2句There are many safety rules that divers must follow to help ensure their survival.可知本段主要讲述的是在探索blue hole的时候要遵循的安全规则。故E项正确。

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