单选题 Geologists have been studying volcanoes for a long time. Though they have learned a great deal, they still have not discovered the causes of volcanic action. They know that the inside of the earth is very hot, but they are not sure exactly what causes the great heat. Some geologists have thought that the heat is caused by great pressure of the earth's outer layers. Or the heat may be left from the time when the earth was formed. During the last sixty veers scientists have learned about radium, uranium, and other radioactive elements. These give out heat all the time as they change into other elements. Many scientists now believe that much of the heat inside the earth is produced by radioactive elements.
Whatever the cause of the beat may be, we do know that the earth gets hotter the farther down we dig. In deep mines and oil wells the temperature rises about 1 °F for each 15 meters. At this rate the temperature 64km below the earth's surface would be over 4,000~F. This is much hotter than necessary to melt rock. However, the pressure of the rock above keeps most materials from melting at their usual melting points. Geologists believe that the rock deep in the earth may be plantic. But if some change in the earth's outer layer releases the pressure, the rock melts. Then the hot, liquid rock can move up toward the surface.
When the melted rock moves with difficulty close to the earth surface, a volcano may be formed. The melted rock often contains steam and other gases under great pressure. If the rock breaks, the pressure is released. Then the sudden expansion of the gases causes explosions. These blow the melted rock into pieces of different sizes and shoot them high in the air.

单选题 The main idea of this passage is ______.

【正确答案】 A

【答案解析】主旨大意题。文章开头即点明了主题：…they still have not discovered the cause of volcanic action(……他们仍没有发现火山活动的成因)。随后，本文分三段分别对地球内部的“great heat”原因及火山最终形成的过程进行了分析。本文中心思想明确，段落结构紧扣主题。显然选项A符合题意，选项B、C、D均为局部细节问题，不属于本文中心思想。

单选题 The cause for the great heat inside the earth is ______.

【正确答案】 C

【答案解析】细节题。第一段第二句讲道：They know that the inside of the earth is very hot, but they are not sure exactly what causes the great heat(他们知道地球内部温度很高，但又不能断定巨热的成因到底是什么)。据此，选项C正确。选项A、B、D中的内容在本段都有描述，但这些只是部分科学家在研究中的发现或推测，并没有得到定论，至于巨热的成因仍然是个谜。据此，上述三项与本题不符。

单选题 Geologists believe that the rock deep in the earth may be ______.

单选题 From the information given in Par

单选题 A volcano may be formed if ______.

【正确答案】 C

【答案解析】细节题。第二段最后两句“But if some change in the earth's outer layer releases the pressure, the rock melts. Then the hot liquid rock can move up toward the surface.” (但是如果地球外层的某种变化释放出压力，岩石就会熔化。然后灼热的岩浆就会喷向表层)讲明了火山的形成过程，与选项C一致。选项A、B、D不是火山形成的条件。