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  3. A meteor stream is composed of dust particles that have been ejected from a

A meteor stream is composed of dust particles that have been ejected from a

游客2025-02-26  9
问题     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 hourse (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.
选项 A、Comparing two scientific theories and contrasting the predictions that each would make concerning a natural phenomenon
B、Describing a new theoretical model and noting that it explains the nature of observations made of a particular natural phenomenon
C、Evaluating the results of a particular scientific experiment and suggesting further areas for research
D、Explaining how two different natural phenomena are related and demonstrating a way to measure them
E、Analyzing recent data derived from observations of an actual phenomenon and constructing a model to explain the data
答案 B
解析 This question asks you to identify the primary focus of the passage. The best answer is B. The author describes the new theoretical model in the first paragraph; in the final paragraph the author states that the data obtained from actual observations, which are discussed in the second and third paragraphs, is consistent with the new theoretical model. Choice A is not correct; the computer model confirmed the astronomers’ hypothesis that meteor streams broaden with time, and although the model yielded an unexpected result, the passage does not contrast the predictions yielded by competing theories. Choice C and D are not correct because the passage makes no reference to further areas for research, and only a single phenomenon is described in the passage. And choice E is not correct because it reverses the order of events. The model yielded a prediction that was subsequently confirmed by observational data, the model was not constructed to explain the data.
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