题目内容

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 differing 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 experiment tested this hypothesis by tracking the influence of planetary gravitation over a projected 5,000-year period on the position 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. Conventional theories, however, predicted that the distribution of particles would be increasingly dense toward the center of a meteor stream. Surprisingly, 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 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.

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第一题 第二题 第三题 第四题 第五题

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 broadened as rapidly as the conventional theories would have predicted.

  • E.

    The computer—model Geminid meteor stream provides an accurate representation of the development of the actual Geminid stream.

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网友讨论

  • whatsup

    A exactly –绝对词语, 最后一句明明说的是”average”
    B. 根本就没有提到相关信息 ---无关
    C. 说的是过去存在多久, 题干说的是未来存在多久, 不对
    D. 第二段说, 地球花1天, 穿越了 stream, 这时候是5000年左右, 那么现在是3000年, 那么时间就会少—19小时. 不会更 boarde
    E. 不要想复杂了 --- 出题点是说:
    现在19小时是我预测的-------->是从 电脑模拟的24小时 5000年推测的------>那么只有5000个小时前提成立, 那么19小时3000年的预测才成立--->计算机得出的

科目:RC阅读

来源:Prep12

难度:700-800

考点:逻辑