A、No more than 1800 years. B、About 2000years. C、Nearly 4000 years. D、Over 5000 years. C

Earthquakes are violent, destructive—and unpredictable! The overall history of earthquakes leads to one general rule: The more time that has passed since an earthquake, the bigger the chance one will strike. Scientists therefore use information about past earthquakes to guess probabilities about future earthquakes. For example, they say that within the next 30 years, there is a 60-percent probability that an earthquake will hit somewhere in southern California. But exactly where or when is a mystery.
    In at least one case, an earthquake in northern China has been predicted by paying attention to clues from nature. In China earthquakes have been studied for nearly 4,000 years, and there are records of more than 9,000 earthquakes going back to the year 1831 B. C. The official earthquake records of the state of California only go back to 1930!
    In their long, hard look at earthquakes, the Chinese have observed many things. Before an earthquake, animals start to act strangely, the ground changes shape, and Earth’s magnetic field gets stronger in the earthquake area. The water in wells bubbles up and then returns to its normal level. Also, a series of small earthquakes can mean that a big quake is on its way.
    The Chinese used these clues in 1974 to make the first successful prediction of an earthquake. The United States Geological Survey, on the other hand, has never predicted a major earthquake.
Question No. 23 What is the general rule about the occurrence of an earthquake?
Question No. 24 According to the speaker, which area is likely to be hit by an earthquake within the next 30 years?
Question No. 25 How long have the Chinese studied earthquakes?
Question No. 26 Which of the following is NOT a clue that is used by the Chinese to predict an earthquake?