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  3. Many of the most damaging and life-threatening types of weather--torrential rains, severe thunderstorms, and tornadoes--begin qu

Many of the most damaging and life-threatening types of weather--torrential rains, severe thunderstorms, and tornadoes--begin qu

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问题    Many of the most damaging and life-threatening types of weather--torrential rains, severe thunderstorms, and tornadoes--begin quickly, strike suddenly, and dissipate rapidly, devastating small regions while leaving neighboring areas untouched. One such event, a tornado, struck the northeastern section of Edmonton, Alberta, in July 1987. Total damages from the tornado exceeded ＄ 250 million, the highest ever for any Canadian storm. Conventional computer models of the atmosphere have limited value in predicting short-lived local storms like the Edmonton tornado, because the available weather data are generally not detailed enough to allow computers to discern the subtle atmospheric changes that precede these storms. In most nations, for example, weather-balloon observations are taken just once every twelve hours at locations typically separated by hundreds of miles. With such limited data, conventional forecasting models do a much better job predicting general weather conditions over larger regions than they do forecasting specific local events.
   Until recently, the observation--intensive approach needed for accurate, very short-range forecasts, or "Nowcasts", was not feasible. The cost of equipping and operating many thousands of conventional weather stations was prohibitively high, and the difficulties involved in rapidly collecting and processing the raw weather data from such a network were insurmountable. Fortunately, scientific and technological advances have overcome most of these problems. Radar systems, automated weather instruments, and satellites are all capable of making detailed, nearly continuous observation over large regions at a relatively low cost. Communications satellites can transmit data around the world cheaply and instantaneously, and modern computers can quickly compile and analyzing this large volume of weather information. Meteorologists and computer scientists now work together to design computer programs and video equipment capable of transforming raw weather data into words, symbols, and vivid graphic displays that forecasters can interpret easily and quickly. As meteorologists have begun using these new technologies in weather forecasting offices, Nowcasting is becoming a reality.  
Why does the author state in the first paragraph that observations are taken "just once every twelve hours"?
通过这个例子，并通过与这个例子相关的以及后续的句子可以得出答案。
选项 A、To indicate that the observations are timely.
B、To show why the observations are of limited value.
C、To compare data from balloons and computers.
D、To give an example of international cooperation.
答案
解析 B
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