单项选择题

案例分析题Since ancient times, the destructive effects of earthquakes on human lives and property have encouraged the search for reliable methods of earthquake prediction. This challenge remains and modern scientists continue to search for reliable methods to determine the time, place and intensity of individual quakes. One prediction technique involves an analysis of the recurrence (重视) rates of earthquakes as indicators of future activity. Earthquakes are concentrated in certain areas of the world which are subject to constant movements of earth’s plates and it is in these areas that scientists focus their investigations.
This search for pre-earthquake phenomena has received particular attention. In contrast to the ancient Greeks and Romans, who relied on the crying of dogs as a warning sign, modern scientists have focused on physical evidence for a coming earthquake. Evidence of plate strain can be found by measuring relative movements, while chemical changes also offer signals for scientists.
Analysis of the changes in magnetic properties and conductivity (传导性) of rocks provides further data for prediction. The electrical and magnetic properties of crustal (地壳的) rocks are particularly sensitive to strain. The conductivity of crustal rock is determined by the degree to which the rock is soaked with fluid and the electrolytic properties of those fluids. Before large earthquakes, small fractures develop in rocks, which change the quantity of fluid present. These changes can be measured and provide useful data. However, similar changes in the fluid-bearing capacity of rock can occur as a result of other factors such as changes in the water table, and therefore this technique is not entirely reliable.
The ancient belief that the behavior of birds, cats and dogs provides evidence of inevitable earthquakes has recently gained credence as a result of tests carried out in California. It has been shown that changes take place in the metabolic (新陈代谢的) rates of these animals which correlate with subsequent seismic activity. It is assumed that the animals are sensitive to the seismic waves which precede major quakes. In zones where earthquakes are known to occur, improved construction techniques can significantly reduce the effects of seismic waves. If more accurate information regarding the time and intensity were available, governments could take even more effective measures to reduce the impact on human lives. If, however, an entirely accurate prediction technique became available, there would be significant social and political implications.
A disadvantage of fluid measuring is that （）.

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