As scientists learn more about the causes of earthquakes, they become better able to predict them. First of all, they know that quakes are likely to occur in areas where the tectonic plates(地层地壳板块)push against each other. They study these areas closely, using different instruments. They measure the vibrations of the earth with seismographs(地震仪), which record on rolls of paper all movements, both large and small. Scientists usually use the Richter scale(里希特震级仪)named after a famous seismologist(地震学家), for these measurements. When an earthquake measures over 4~5 on the Richter scale, it is strong enough to cause damage. Scientists have determined that an area is not likely to have a large earthquake when many small quakes have been occurring. When the small quakes stop, then pressure builds and a strong quake is more likely.
　　There are several other ways to predict earthquakes. Scientists can measure the tiny changes in the tilt(倾斜)of the land that happen before earthquakes occur. Increases in the amount of radon(氡), a substance found in deep wells, as well as a higher level of water in the wells, can also mean that an earthquake is likely to occur. In addition, scientists have found that some animals, like dogs, behave strangely before an earthquake, they become nervous and howl(号叫). Seismologists have correctly predicted several earthquakes. In 1975, they said that an earthquake would occur in Liaoning Province, China, one month, and then one hour, before it happened. In 1978, Russian scientists predicted an earthquake in the Altai Mountains six hours before it occurred. However, not all earthquakes are predictable, and a system for predicting is not yet perfected. Nothing on this earth is certain. Although people who live in California and Japan live with the possibility of a serious quake, perhaps the next strong earthquake will occur somewhere outside the "Ring of Fire".
　　21. According to the information in Para. 1 seismograph is ______.
　　A. a computer like electronic instrument for predicting earthquakes
　　B. a special instrument for recording and measuring the vibrations of the earth
　　C. an instrument which seismologists use to make studies on the causes of earthquakes
　　D. an instrument with which seismologists measure the pressure between tectonic plates
　　答案：B
　　22. According to the passage, a large earthquake is most likely to occur in an area ______.
　　A. where a series of minor vibrations of the earth have been occurring
　　B. that is within the "Ring of Fire" but does not see any vibration of the earth
　　C. where many small vibrations of the earth have been occurring and then a calm period follows
　　D. where tectonic plates fit into each other properly
　　答案：C
　　23. The following signs that show the coming of an earthquake are mentioned in the passage EXCEPT that ______.
　　A. the amount of radon in deep wells increases
　　B. the earth gives off flashes of light
　　C. the level of water in the wells rises
　　D. some animals become nervous and behave strangely
　　答案：B
　　24. According to the passage, the earthquake occurred in China’s Liaoning Province in 1975 ______.
　　A. was twice predicted by seismologists
　　B. took all the people in the area by surprise
　　C. was followed by many small vibrations of the earth
　　D. was correctly predicted by Russian seismologists
　　答案：A
　　25. Which of the following statements is TRUE according to the passage?
　　A. Not all earthquakes are predictable although seismologists have correctly predicted some.
　　B. Seismology was fully developed now and earthquake prediction is always reliable.
　　C. No earthquake prediction is reliable since nothing on this earth is certain.
　　D. A system for predicting the earthquake is perfected, but prediction is not always correct.
　　答案：A

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