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  3. It is now well-established that petroleum migrates through aquifers and can become trapped in reservoirs. Petroleum migration is

It is now well-established that petroleum migrates through aquifers and can become trapped in reservoirs. Petroleum migration is

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问题     It is now well-established that petroleum migrates through aquifers and can become trapped in reservoirs. Petroleum migration is analogous to groundwater migration. When oil and gas are squeezed out of the shale in which they originated and enter a body of sandstone or limestone somewhere above, they migrate readily because sandstone (consisting of quartz grains) and limestone (consisting of carbonate minerals) are much more permeable than any shale. The force of molecular attraction between oil and quartz or carbonate minerals is weaker than that between water and quartz or carbonate minerals. Hence, because oil and water do not mix, water remains fastened to the quartz or carbonate grains, while oil occupies the central parts of the larger openings in the porous sandstone or limestone. Because oil is lighter than water, it tends to glide upward past the carbonate- and quartz-held water. In this way, oil becomes segregated from the water; when it encounters a trap, it can form a pool.
    Physiological adaptations can assist amphibians in colonizing habitats where extreme conditions prevail. The tolerance range in body temperature represents the range of temperatures within which a species can survive. One species of North American newt is still active when temperatures drop to -2℃, while one South American frog feels comfortable even when temperatures rise to 41℃—the highest body temperature measured in a free-ranging amphibian. Recently it has been shown that some North American frog and toad species can survive up to five days with a body temperature of -6℃ with approximately one third of their body fluids frozen. The other tissues are protected because they contain the frost-protective agents glycerin or glucose. Additionally, in many species, the tolerance boundaries are flexible and can change as a result of acclimatization (long-term exposure to particular conditions).
    While many other observers and thinkers had laid the groundwork for science, Thales (circa 624 B.C.— circa 547 B.C.), the best known of the earliest Greek philosophers, made the first steps toward a new, more objective approach to finding out about the world. He posed a very basic question: "What is the world made of?" Many others had asked the same question before him, but Thales based his answer strictly on what he had observed and what he could reason out—not on imaginative stories about the gods or the supernatural. He proposed water as the single substance from which everything in the world was made and developed a model of the universe with Earth as a flat disk floating in water.
Why does the author mention a "South American frog" species in the passage?
选项 A、To make the point that an amphibian’s temperature tolerance depends on a number of factors
B、To indicate how precise the range of body temperatures is for certain amphibians
C、To contrast its ability to adapt to that of the North American newt
D、To help illustrate the range of environmental conditions to which amphibians have adapted
答案B
解析
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