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题库 / Prep2012-RC-9
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.
The passage suggests that which of the following is a prediction concerning meteor streams that can be derived from both the conventional theories mentioned in the highlighted text and the new computer derived theory?
Dust particles in a meteor stream will usually be distributed evenly throughout any cross section of the stream.
The orbits of most meteor streams should cross the orbit of the Earth at some point and give rise to a meteor shower.
Over time the distribution of dust in a meteor stream will usually become denser at the outside edges of the stream than at the center.
Meteor showers caused by older meteor streams should be, on average, longer in duration than those caused by very young meteor streams.
The individual dust particles in older meteor streams should be, on average, smaller than those that compose younger meteor streams.
此讲解的内容由AI生成，还未经人工审阅，仅供参考。
正确答案是 B。
B 选项正确，原因是文中提到，在传统理论下，当地球通过一个流星流时，会引发流星雨事件。此外，根据文章中最后一段介绍的电脑建模实验，可以得出相应的结论：Geminid流星流的前5 000 年会产生一个被地球穿过的“壳状空心管”，这时引发的流星雨将会出现双峰状的活动，符合 B 选项的描述。
5000年的流星雨穿过要一天多，3000年的流星雨穿过19小时，这是客观事实，不是争议点，争议的地方是到底是流星管子中间密度大还是两边密度大，所以两个是都同意的。唉。。。。。5000年的风风雨雨藏聊多少痛。。。
最后一句跟着唱了起来 哈哈哈哈
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选项D说新旧理论都同意older streams流星雨时间更长，可是conventional theories的理论明明是particles会变得更紧密increasingly dense toward the center of a meteor stream，因此时间流星雨时间会变短才对？陷入了奇怪的思考回路 求解析
Inference
The question's use of the word suggests means that the answer depends on making an
inference. The third sentence of the passage states the hypothesis that a meteor stream
should broaden with time; this hypothesis is consistent with both conventional and
computer-derived theories regarding the nature of the center of the meteor stream. Thus,
the broader the meteor stream is, the older it is. The first sentence of the second paragraph
states that meteor showers occur whenever Earth passes through a meteor stream. It can be
inferred that if the meteor stream is older and broader, Earth will experience longer periods
of meteor showers as it passes through this broad stream than it would if the meteor stream
were younger and therefore less broad.
争议点：
Conventional theory: the distribution of particles would be increasingly dense toward the center of a meteor stream
computer model: the computer-model meteor stream gradually came to resemble a thick-walled, hollow pipe
既定事实：The researcher found, as expected, that the computer-model stream broadened with time.
The researcher found, as expected, that the computer-model stream broadened with time. 时间越长越宽！
conventional: Astronomers have hypothesized that a meteor stream should broaden with time...
computer theory: The researcher found, as expected, that the computer-model stream broadened with time
共同点就是meteor shower随着时间变宽。-> 地球穿过older meteor的时间longer than the young ones
原文“The time intervals between the bursts ”，问题duration，可能burst之间仍然是duration吧，造成不敢选C。内心觉得题目要考的就是时间的问题，
注意D选项的比较对象是新老meteor streams. 不是在new computer derived theory和 conventional theories中的 。
5,000 years old.---24h to cross
3000 ----19h
看明白了。。The researcher found, as expected, that the computer-model stream broadened with time. ←时间越长stream越宽是客观事实。理论的争议是在粒子分布的密度。
二者对the distribution of particles有不同观点
over a day对应5000years old；19 hours对应3000years old 所以D
时间更长
流星流随着时间会扩张变大，地球以恒定的时间穿过老的流星流由于距离更大，所以时间更短
兩個理論都同意的地方，即事實的存在
Conventional theories：dense toward the center 与C答案相违背