题文
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. 题型：未知难度：其他题型
答案
小题1:F
小题2:C
小题3:A
小题4:B
小题5:E
点击查看序数词知识点讲解,巩固学习
解析
文章介绍了探索地球上神秘的蓝洞，包括蓝洞的定义，蓝洞是怎么形成的，蓝洞的不同的结构等。
小题1:考查段落大意和句意理解：根据这段的句子：A blue hole is a flooded sea cave with a hole that opens up at the land's surface.可知这段讲的是：蓝洞的大致的定义。答案是F
小题2:考查段落大意和句意理解：根据这段的句子：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.可知这段讲的是：蓝洞是怎么形成的，答案是C
小题3:考查段落大意和句意理解：根据这段的句子：They have several layers of water, chemicals, and bacteria.可知这段讲的是：蓝洞的不同的结构，答案是A
小题4:考查段落大意和句意理解：根据这段的句子：Scientists are interested in these caves because oxygen-free conditions there are similar to those on Earth long ago,可知这段讲的是：为什么科学家对蓝洞感兴趣，答案是B
小题5:考查段落大意和句意理解：根据这段的句子：There are many safety rules that divers must follow to help ensure their survival.可知这段讲的是：潜入蓝洞要注意的规定，答案是E
考点
据考高分专家说，试题“根据短文内容，从下边的A—F选项中选出能.....”主要考查你对 [序数词 ]考点的理解。序数词
序数词的概念：
表示顺序的数称为序数词。如：first, second, third, fourth。
序数词的构成与用法：
1、序数词的构成：
①一般来说，是由相应的基数词加词尾th构成。
例：four＋th→fourth
six＋th→sixth
seven＋th→seventh
ten＋th→tenth
②下面这些基数词在变为序数词时，有特殊的变化。
例：one→first
two→second
three→third
five→fifth
eight→eighth
nine→ninth
twelve→twelfth
③十位整数序数词的构成方法是将基数词的词y变成i，然后再加eth。
例：twenty→twentieth
thirty→thirtieth
forty→fortieth
ninety→ninetieth
④两位或两位以上的基数词变成序数词时，仅将个位数变成序数词。
例：twenty－one→twenty－first
thirty－five→thirty－fifth
a hundred and fifty－three→a hundred and fifty－third
2、序数词的用法：
①序数词在使用时，一般加上定冠词。
例：the first book
the second floor
the third day
the fourth week．
②序数词在多数情况下都用作定语，有的也可以作表语、主语和宾语。
例：The may1st is Labour Day. 五月一日是劳动节。
My room is on the second floor. 我的房间在二楼。
The first is larger than the secon.（主语）第一个比第二个大。
Read the book from the first.（宾语）从开头读这本书。
You'll be the sixth to write.（表语）你将是第六个写的。
③序数词的前面可以加上不定冠词，用来表示“再一”，“又一”的意思。
例：You may have a third try. 你可以第三次尝试。
序数词知识体系：
约数的表达方法：
用tens/dozens/scores/hundreds/thousands/millions of 表示“几十、几百、上千、成千上万”等。
如：The boy bought dozens of pencils.
Thousands of people died in the earthquake.
注意： (A)：dozen, score, hundred, thousand, million等表示确切数量时，不用复数。
如：five dozen (of) eggs 五打鸡蛋
hree hundred people 三百个人
分数词的构成和用法：
1）分数词构成法：
分数词（FractionalNumerals）由基数词和序数词构成，基数词代表分子，序数词代表分母。除了分子为1的情况下，序数词都要用复数形式：
如：1/4：one-fourth
5/9：five-ninths
2/3：two-thirds
17/5：three and two-fifths
7/12：seven-twelfths
379/8：forty-seven and three-eighths
此外还有下面表示法：
如：1/2：a(one) half
1/4：a(one) quarter
3/4：three-quarters
9/4：two and a quarter
3/2：one and half
31/4：seven and three quarters
