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  3. Before 1965 many scientists pictured the circulation of the ocean’s water mass as consisting of large slow-moving currents, such

Before 1965 many scientists pictured the circulation of the ocean’s water mass as consisting of large slow-moving currents, such

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问题      Before 1965 many scientists pictured the circulation of the ocean’s water mass as consisting of large slow-moving currents, such as the Gulf Stream. That view, based on 100 years of observations made around the globe, produced only a rough approximation of the true circulation. But in the 1950s and the 1960s, researchers began to employ newly developed techniques and equipment including subsurface floats that move with ocean current and emit identification signals for months at fixed locations in the ocean. These instruments disclosed an unexpected level of variability in the’ deep ocean. Rather than being characterized by smooth, large-scale currents that change seasonally (if at all), the seas are dominated by what oceanographers (海洋研究者) call mesoscale fields (紊流): fluctuating, energetic flows whose velocity can reach ten times of the mean velocity of the majior currents.
     Mesoscale phenomena--the oceanic analogue of weather terms--often extend to distances of 100 kilometers and persist for 100 days (weather systems generally extend about 1 000 kilometers and last 3 to 5 days in any given area). More than 90 percent of the kinetic energy of the entire ocean may be accounted for by mesoscale variability rather than by large-scale currents. Mesoscale phenomena may, in fact, play a significant role in oceanic mixing air-sea interactions, and’ occasional but far-reaching climate events such as EI Nino ,the atmospheric-oceanic disturbance in the equatorial Pacific that affects global weather patterns.
     Unfortunately, it is not feasible to use conventional techniques to measure mesoscale fields. To measure them properly, monitoring equipments would have to be laid out on a grid at intervals of at most 50 kilometers, with sensors at each grid point lowered deep in the ocean and kept there for many months. Because using these techniques would be prohibitively expensive and time-consuming, it was proposed in 1975 that tomography (X射线断层摄影技术) be adapted to measuring the physical properties of the ocean. In medical tomography X-rays map the human body’s density variations (and hence internal organs); the information from the X-rays, transmitted through the body along many different paths, is recombined to form three-dimensional images of the body’s interior. It is primarily this multiplicative increase in data obtained from the multi-path transmission of signals that accounts for oceanographers’ attraction to tomography: it allows the measurement of vast areas with relatively few instruments. Researchers reasoned that low-frequency sound waves ,because they are so well described mathematically and because even small perturbations(动摇) in emitted sound waves can be detected, could be transmitted through the ocean over many different paths and that the properties of the ocean’s interior-its temperature, salinity, density and speed of currents-could be deduced on the basis of how the ocean altered the signals. Their initial trials were highly successful, and ocean acoustic tomography was born.  
Which of the following, if presented as the first sentence of a succeeding paragraph, would most logically continue the discussion presented in the passage?
选项 A、Timekeeping in medical tomography must be precise because the changes in travel time caused by density fluctuations are slight.
B、To understand how ocean acoustic tomography works, it is necessary to know how sound travels in the ocean.
C、Ships are another possibility, but they would need to stop every 50 kilometers to lower measuring instruments.
D、These variations amount to only about 2 to 3 percent of the average speed of sound in water, which is about i 500 meters per second.
答案B
解析 推断题。文章节选部分第三段最后一句指出，“海洋声音X线断层摄影术就诞生了”，顺理成章，下文应继续介绍这种技术的原理及具体应用．只有B项“要想了解海洋声音X线断层摄影术是如何工作的，就需要知道声音是如何在海洋中传播的”，介绍其工作原理，符合顺承文意的要求。
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