求教各位大牛！Prep 2012的一篇文章

chenhao6789

有一道题怎么看不懂 找不到题目中说的两种观点的共同点.....求指导! 是Pack里的第四篇..全文还有题目如下：
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.

PREP2012-Pack1-RC-002-04 VRC07544-04 Hard


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 text
and the new computer derived theory?

A. Dust particles in a meteor stream will usually be distributed evenly throughout any cross
section of the stream.

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 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.

感激不尽！