上午做了一篇特让人郁闷的RC,做了好几次都错了，说说你们做这篇文章的感觉吧？好几题我是连定位的思路都没有A meteor stream is composed of dust particles that have been ejected from a parent comet at a variety of velocities. These particles follow the same orbit as the parent comet, but due to their differeing velocities they slowly gain on or fall behind the disintegrating comet until a shroud of dust surrounds the entire cometary orbit. Astronomers have hypothesized that a meteor stream should broaden with time as the dust particles’ individual orbits are perturbed by planetary gravitational fields. A recent computer-modeling experimetn tested this hypothesis by tracking the influence of planetary gravitation over a projected 5,000-year period on the positions of a group of hypothetical dust particles. In the model, the particles were randomly distributed throughout a computer simulation of the orbit of an actual meteor stream, the Geminid. The researcher found, as expected, that the computer-model stream broadened with time. Coventional theories, however, predicted that the distribution of particles would be increaingly dense toward the center of a meteor stream. Surpringly, the computer-model meteor stream gradually came to resemble a thick-walled, hollow pipe. Whenever the Earth passes through a meteor stream, a meteor shower occurs. Moving at a little over 1,500,000 miles per day around its orbit, the Earth would take, on average, just over a day to cross the hollow, computer-model Geminid stream if the stream were 5,000 years old. Two brief periods of peak meteor activity during the shower would be observed, one as the Earth entered the thick-walled “pipe” and one as it exited. There is no reason why the Earth should always pass through the stream’s exact center, so the time interval between the two bursts of activity would vary from one year to the next. Has the predicted twin-peaked activity been observed for the actual yearly GEminid meteor shower? The Geminid data between 1970 and 1979 show just such a bifurcation, a secondary burst of meteor activity being clearly visible at an average of 19 hours (1,200,000 miles) after the first burst. The time intervals between the bursts suggest the actual Geminid stream is about 3,000 years old.
86. It can be inferred from the last paragraph of the passage that which of the following must be true of the Earth as it orbits the Sun? (A) Most meteor streams it encounters are more than 2,000 years old. (B) When passing through a meteor stream, it usually passes near to the stream’s center. (C) It crosses the Geminid meteor stream once every year. (D) It usually takes over a day to cross the actual Geminid meteor stream. (E) It accounts of msot of the gravitaitonal perturbation affecting the Geminid meteor stream.
This question asks you to draw an inference from information in the last paragraph of the passage. · The best answer is C. According to the passage, the Geminid meteor shower occurs yearly; because meteor showers occur whenever the Earth passes through a meteor stream, one can infer that the Earth crosses the Geminid stream once every year. · Choice A is incorrect because the passage provides no information from which to generalize about the age of meteor streams. Choice B, which is directly contradicted by lines 31-32, is also incorrect. · Choice D is incorrect. In lines 25-28, the passage says that the Earth would take just over a day to cross the stream if the stream were 5,000 years old. However, in lines 38-42 the passage states that in fact an average of only 19 houses elapsed between the time that the Earth entered the stream until the time that it exited, leading researchers to conclude that the stream is only about 3,000 years old. · Choice E is incorrect because the passage says only that planetary gravitational fields perturb the orbits of dust particles in a meteor stream; it does not say that the effect of the Earth’s gravitation is greater than that of other planets.
87. Which of the following is an assumption underlying the last sentence of the passage? (A) In each of the years between 1970 and 1979, the Earth took exactly 19 hours to cross the Geminid meteor stream. (B) The comet associated with the Geminid meteor stream has totally disintegrated. (C) The Geminid meteor stream should continue to exist for at least 5,000 years. (D) The Geminid meteor stream has not broadended as rapidly as the conventiona ltheories would have predicted. (E) The computer-model Geminid meteor stream provides an accurate representation of the development of the actual Geminid stream.
This question asks you to identify an assumption underlying the last sentence of the passage. In this sentence, the author of the passage draws a conclusion about the age of the Geminid stream. This conclusion is based on two pieces of information. The first is the length of time the Earth would take to cross the computer-model Geminid stream if the stream were 5,000 years old (lines 24-28). The second is the actual elapsed time between the two peaks of meteor activity predicted by the computer model (lines 36-40). In concluding from this information that the Geminid stream is actually only 3,000 years old, the author is assuming the accuracy of the computer model, as stated in E, the best answer. · Choice A is incorrect because the passage says that the time the Earth takes to cross the stream would vary from year to year (lines 32-34) and that 19 hours was the average time, not the exact time, observed from 1970 to 1979 (lines 36-40). · Choices B and C are incorrect because the passage does not suggest anything about the current state of the comet associated with the Geminid stream or about the expected longevity of the stream. · Choices D is incorrect because the computer model is said to confirm the broadening predicted by the conventional theories; the fact that the model projected the positions of the particles in the stream over a 5,000-year period does not suggest that researchers expected the stream to be older (and therefore broader) than it turned out to be.  | 
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发表于 2007-2-23 12:22:00
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