og 85

alohame

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.

85. 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 line 18 and the new computer-derived theory?
(A) Dust particles in a meteor stream will usually be distributed evenly throughout any cross section of the steam.
(B) The orbits of most meteor streams should cross the orbit of the Earth at some point and give rise to a meteor shower.
(C) 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.
(D) Meteor showers caused by older by older meteor streams should be, on average, longer in duration than those caused by very young meteor streams.
(E) The individual dust particles in older meteor streams should be, on average, smaller than those that compose younger meteor streams.

This question asks you to identify a prediction that can be derived from both the conventional
theories about meteor streams and the new computer-derived model. You must base your answer
on information that is suggested by, but not expressly stated in, the passage.
According to lines 7-8 of the passage, the conventional theories hypothesized that meteor streams
should broaden with time, and the computer simulation confirmed this hypothesis. The passage
also suggests that the time it takes for the Earth to cross a meteor stream (and, by implication, the
duration of the resulting meteor shower) is directly related to the breadth of the stream (lines
23-28). From these pieces of information, which are supported by both the conventional theories
and the new computer-derived theory, it can be inferred that on average the meteor showers
caused by older (and therefore broader) meteor streams would be longer in duration than those
caused by very young (and therefore narrower) meteor streams, as stated in D, the best answer.

答案没问题，但是这个题干明明说的是18行蓝色的conventional theory，却定位在橙色的astronomer...上，ETS这是作何居心！astronomer说的话怎能平白无故的变成理论了？

[此贴子已经被作者于2006-9-17 17:01:29编辑过]

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