[求助]OG#14一题~！！

rea89

2772#(2/17)

A meteor stream is composed of dust particles that havebeen ejected from
a parent comet at a variety of velocities. These particlesfollow the same orbit
as the parent comet, but due to their differingvelocities they slowly gain on or
fall behind the disintegrating cometuntil a shroud of dust surrounds the entire
cometary orbit. Astronomers havehypothesized that a meteor stream should
broaden with time as the dustparticles’ individual orbits are perturbed by
planetary gravitational fields. Arecent computer-modeling experiment tested
this hypothesis by tracking theinfluence of planetary gravitation over a
projected 5,000-year period on thepositions of a group of hypothetical dust
particles. In the model, theparticles were randomly distributed throughout a
computer simulationof the orbit of an actual meteor stream,the Geminid.
The researcher found, as expected, that the computer-model streambroadened
with time. Conventional theories, however, predicted that thedistribution of
particles would be increasingly dense toward the center of ameteor stream.
Surprisingly, the computer-model meteor stream gradually came toresemble
a thick-walled, hollow pipe.


Whenever the Earth passes through a meteor stream, ameteor shower
occurs. Moving at a little over1,500,000 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 thestream were 5,000 years old. Two brief
periods of peak meteor activity duringthe 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 passthrough the stream’s exact center, so the time interval
between the two burstsof activity would vary from one year to the next.


Has the predicted twin-peaked activity been observedfor the actual yearly Geminid
meteor shower? The Geminid data between 1970 and1979 show just such a
bifurcation,a secondary burst of meteor activity beingclearly visible at an average
of 19 hours (1,200,000 miles) after the firstburst. The time intervals between the
bursts suggest the actual Geminid streamis about 3,000 years old.

rea89

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 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.

这是选项。答案是D。但是我貌似定位不了撒。困惑啊