求助一道OG阅读题，思维障碍了。。。多谢

misscou

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 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.

第一段，计算机模拟证实，meteor stream 是随着时间变宽的，但是和传统理论不一样的是，meteor stream 类似一个空心的管子。
第二段，在寻找实证 来推断metero stream 是实心的还是空心的。实证是：地球经过 meteor stream 就一定会有meteor shower。地球的行进速度已知，推测差不多一天的时间，穿过meteor stream. 但是，不保证每次地球都是恰好从meteor stream 的中心穿过，所以穿过的时间是不一样。
第三段，实证 证明了两点，1 真是空心的，2 随着时间的增加，meteor stream 是会变宽的。

所以我想问85题D选项（D为正确选项），我怎么感觉因为随着时间推移stream会broaden,所以应该是越老的stream产生的shower时间更短吧。。。求大牛解释，多谢。。。

歪歪的

我觉得是shower的时间应该是从一开始进入到最后出去之间的时间。(prep解释中有from start to finish这句话）因为虽然像一个空心的管子，但是我认为分布是呈现边缘更密集的趋势，中间还是有的，不然也不会说是peak。因此broaden了时间就更短了。不知道有没有帮助到lz。
我可以说我第一次看这文章没抓住中心么╮(╯▽╰)╭