一篇文章，或是陈述一个事实，或是解释一个问题，或是论证一个道理，这就是作者（或文章）的意图。首先我们来看一看问题的形式：
IV.PURPOSE
1. What is the purpose of the passage?
2. What is the main purpose of the passage?
3. What is the author's main purpose in the passage?
4. The point the writer is making in the passage is that _____.
5. The purpose of this passage is to _______.
6. The author probably intended this selection to ______.
在解答这类问题时，重点在于分析文章的思路，分析作者是如何加工素材的，加工素材的步骤和走向又如何。下面我们来看一篇文章.
IVA.
Convective flow should be familiar to anyone who has noted the boiling of a heated liquid. The most elementary type of convection can be explained by the fact that heat rises. In the simplest cases, convective flow begins
when a fluid is heated from below. As the bottom layer of the fluid is heated, it expands and thus becomes less dense than the layers above. The warmer and lighter bottom layer then tends to rise and the cooler layer
tends to sink in a continuous cycle. The same mechanism of convective flow s responsible for the great ocean currents and for the global circulation of the atmosphere. In an ocean, the water is warmed by the sun to a depth of perhaps thirty meters, and evaporation near the water's surface is responsible for the cooling effect.
The main purpose of this passage is to _______.
A. explain the basic principle of convection
B. describe regular changes in the Earth's atmosphere
C. explain the boiling temperatures of liquids
D. state the principles of ocean current
这篇文章讲述的是对流convective flow。仔细分析一下作者的思路和选材，第一句话是引导句，从第二句话开始，就以液体为例，讲述对流的机理mechanism of convective flow,所以作者的目的就是解释对流的原理，答案应该选A.可见，在解答此类问题时，和前面一样，还是要设法抓住主题句。在看一个例子：
IVB.
Although commonly thought to be one of man's feared deepsea enemy, the octopus(章鱼) is generally a harmless animal that rarely bothers man. In fact, most of this powerful, eightarmed creatures are afraid of people.
There have been some cases in which octopuses have attacked divers. But even these scattered attacks have not been very serious. In the mid nineteenth century, Hugo is said to have started the idea that the octopus
is a vicious monster of the deep. In this "Toilers of the Sea" he described how this devil fish eats a human being. The tale became so popular that other novelists, and later the movies used episodes which describe man struggling in the arms of this frightening monster. And thus, the misconception of the octopus as a vicious, merciless killer was spread.
The purpose of this passage is to ______.
A. describes the stories of the octopus
B. explain the causes of the misconception of the octopus
C. prove the harmlessness of the octopus
D. correct the misconception of the octopus
这篇文章的题目出的很好，有较强的干扰性，正确答案应该是D, 有没有选对？下面我们来分析一下。我想这篇文章可以分成三个部分：
1. 第一句话
2. from "In fact,..." to "...very serious".
3. from "In the mid..." to end.
第一部分显然是主题句, 第二部分证明章鱼是无害的, 第三部分解释对章鱼偏见的来由.选项中B 和C 只是概括了文章的部分内容, 只有D才是最符合文章主题句的.最后一篇文章供你们练习,有点难度,但只要抓住主题句,还是能够选对的.
IVC.
Astronomers say Arizona's observatories are being jeopardized by the state's burgeoning population because artificial lights glow steadily brighter and make the stars harder to see. Research has already been
severely impaired a the Lowell observatory in Flagstaff, Arizona, by the lights of the encircling city of 39,000. Each year, the observatory's telescopes find it more difficult to penetrate the artificial lights. As a result, astronomers are working with local officials to dim the glow with restrictions on searchlights and requirements for light shields. Mercury-based streetlights, the most common type in cities in the United
States, are especially disturbing to stargazers (天文学家). An alternative would be sodium lights, which emit no ultraviolet (紫外线的) waveband free the ultraviolet band that is essential to spectral astronomy.
What is the purpose of the passage?
A. To explain recent research
B. To advertise for better telescopes
C. To criticize current lighting system
D. To report on an existing situation
答案是D, 主题句是第一句话。
