It is a fundamental tenet of geophysics that the Earth's magnetic field can exist in either of two polarity states: a "normal" state, in which north-seeking compass needles point to the geographic north, and a "reverse" state, in which they point to the geographic south. Geological evidence shows that periodically the field's polarity reverses, and that these reversals have been taking place at an increasing rate. Evidence also indicates that the field does not reverse instantaneously from one polarity state to another; rather, the process involves a transition period that typically spans a few thousand years.
Though this much is known, the underlying causes of the reversal phenomenon are not well understood. It is generally accepted that the magnetic field itself is generated by the motion of free electrons in the outer core, a slowly churning mass of molten metal sandwiched between the Earth's mantle (the region of the Earth's interior lying below the crust) and its solid inner core. In some way that is not completely understood, gravity and the Earth's rotation, acting on temperature and density differences within the outer core fluid, provide the driving forces behind the generation of the field. The reversal phenomenon may be triggered when something disturbs the heat circulation pattern of the outer core fluid, and with it the magnetic field.
Several explanations for this phenomenon have been proposed. One proposal, the "heat-transfer hypothesis. " ss that the triggering process is intimately related to the way the outer core vents its heat into the mantle. For example, such heat transfer could create hotter (rising) or cooler (descending )blobs of material from the inner and outer boundaries of the fluid core, thereby perturbing the main heat-circulation pattern. A more controversial alternative proposal is the asteroid-impact hypothesis.? In this scenario an extended period of cold and darkness results from the impact of an asteroid large enough to send a great cloud of dust into the atmosphere. Following this climatic change, ocean temperatures drop and the polar ice caps grow, redistributing the Earth's seawater. This redistribution increases the rotational acceleration of the mantle, causing friction and turbulence near the outer core-mantle boundary and initiation reversal of the magnetic field.
How well do these hypotheses account for such observations as the long-term increase in the frequency of reversal? In support of the asteroid-impact model, it had been argued that the gradual cooling of the average ocean temperature would enable progressively smaller asteroid impacts (which are known to occur more frequently than larger impacts) to cool the Earth's climate sufficiently to induce ice-cap growth and reversals. But theories that depend on extraterrestrial intervention seem less convincing than theories like the first, which account for the phenomenon solely by means of the thermodynamic state of the outer core and its effect on the mantle.
2. The author's objection to the second hypothesis discussed in the passage is most applicable to which one of the following explanations concerning the extinction of the dinosaurs?
(A) The extinction of the dinosaurs was the result of gradual changes in the composition of the Earth's atmosphere that occurred over millions of years.
(B) The dinosaurs became extinct when their food supply was disrupted following the emergence of mammals.
(C) The dinosaurs succumbed to the new, colder environment brought about by a buildup of volcanic ash in the atmosphere.
(D) After massively overpopulation the planet, dinosaurs disappeared due to widespread starvation and the rapid spread of disease.
(E) After radical climatic changes resulted from the impact of a comet, dinosaurs disappeared from the Earth.
key's E.文中对2nd hypothesis即asteroid-impact hypothesis的反对，就是最后一段里面讲的“In support of the asteroid-impact model, it had been argued that the gradual cooling of the average ocean temperature would enable progressively smaller asteroid impacts (which are known to occur more frequently than larger impacts) to cool the Earth's climate sufficiently to induce ice-cap growth and reversals. But theories that depend on extraterrestrial intervention seem less convincing than theories like the first,...”就是说发生这种外星物体碰撞情况的可能性比较小。运用到恐龙灭绝怎么会是E呢？E正好是2nd hypothesis嘛。排除了BCD，就还剩A了。
to pocahontas,这里是讨论逻辑的，将阅读放到阅读区好点。
你的问题关键是对问题的理解：问题是说，作者对第二个假设的反对（理由）也能用于下面对恐龙的灭绝的解释。作者反对第二假设的原因是 depend on extraterrestrial intervention。E也提到过外星因素（impact of a comet）。阅读思路和逻辑思路很不同，阅读不需严密，只要回答问题，原文有着意思就行。
啊，这样啊。。。也对。=p
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