主旨题型的标志：出现mainly about； best title； purpose等这样的单词
如2001年第三篇文章开头直接出现了问题Why do so many Americans distrust what they read in their newspapers？为什么美国人不相信他们在报纸上所读到东西？那么原因就是主题，选项B就是正确答案。
59. What is the passage mainly about？
[A] needs of the readers all over the world
[B] causes of the public disappointment about newspapers
[C] origins of the declining newspaper industry
[D] aims of a journalism credibility project
Scattered around the globe are more than 100 small regions of isolated volcanic activity known to geologists as hot spots. Unlike most of the world's volcanoes， they are not always found at the boundaries of the great drifting plates that make up the earth's surface； on the contrary， many of them lie deep in the interior of a plate. Most of the hot spots move only slowly， and in some cases the movement of the plates past them has left trails of dead volcanoes. The hot spots and their volcanic trails are milestones that mark the passage of the plates.
That the plates are moving is now beyond dispute. Africa and South America， for example， are moving away from each other as new material is injected into the sea floor between them. The complementary coastlines and certain geological features that seem to span the ocean are reminders of where the two continents were once joined. The relative motion of the plates carrying these continents has been constructed in detail， but the motion of one plate with respect to another cannot readily be translated into motion with respect to the earth's interior. It is not possible to determine whether both continents are moving in opposite directions or whether one continent is stationary and the other is drifting away from it. Hot spots， anchored in the deeper layers of the earth， provide the measuring instruments needed to resolve the question. From an analysis of the hot-spot population it appears that the African plate is stationary and that it has not moved during the past 30 million years.
The significance of hot spots is not confined to their role as a frame of reference. It now appears that they also have an important influence on the geophysical processes that propel the plates across the globe. When a continental plate come to rest over a hot spot， the material rising from deeper layer creates a broad dome. As the dome grows， it develops deep fissures （cracks）； in at least a few cases the continent may break entirely along some of these fissures， so that the hot spot initiates the formation of a new ocean. Thus just as earlier theories have explained the mobility of the continents， so hot spots may explain their mutability （inconstancy）。
70. The passage is mainly about ________.
[A] the features of volcanic activities
[B] the importance of the theory about drifting plates
[C] the significance of hot spots in geophysical studies（出现中心词hot spot，正确答案）
[D] the process of the formation of volcanoes