[求助]OG 14-86

Passage 14

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[y1] comet until a shroud[y2] 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 perturbe[y3] d by planetary[y4] 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 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.

86. It can be inferred from the last paragraph of the passage that which of the following must be true of the Earth as it orbits the Sun?

(A) Most meteor streams it encounters are more than 2,000 years old.

(B) When passing through a meteor stream, it usually passes near to the stream’s center.

(C) It crosses the Geminid meteor stream once every year.

(D) It usually takes over a day to cross the actual Geminid meteor stream.

(E) It accounts of msot of the gravitaitonal perturbation affecting the Geminid meteor stream.

OG 解释

This question asks you to draw an inference from information in the last paragraph of the passage.

·       The best answer is C. According to the passage, the Geminid meteor shower occurs yearly; because meteor showers occur whenever the Earth passes through a meteor stream, one can infer that the Earth crosses the Geminid stream once every year.

·       Choice A is incorrect because the passage provides no information from which to generalize about the age of meteor streams. Choice B, which is directly contradicted by lines 31-32, is also incorrect.

·       Choice D is incorrect. In lines 25-28, the passage says that the Earth would take just over a day to cross the stream if the stream were 5,000 years old. However, in lines 38-42 the passage states that in fact an average of only 19 houses elapsed between the time that the Earth entered the stream until the time that it exited, leading researchers to conclude that the stream is only about 3,000 years old.

·       Choice E is incorrect because the passage says only that planetary gravitational fields perturb the orbits of dust particles in a meteor stream; it does not say that the effect of the Earth’s gravitation is greater than that of other planets.

两个问题

1 请看黄色加亮部分：第一句说地球用一天时间经过流星中空部分，第二句说地球经过流行边缘所造成的两次流星雨至少要一年。我的理解是地球经过中空部分的时间和两次流星雨的时间应该是一样的吧？请大家看看我的理解是否有问题

2 可能是因为问题一没有搞懂，所以86题C选项怎么也推不出来，请指点

1。为什么经过中空部分的时间要和经过两边部分一样？偶不是很理解Monica的思路的说

2。http://forum.chasedream.com/dispbbs.asp?boardid=25&replyid=24043&id=24043&page=1&skin=0&Star=3

我这篇也是看的懵懵懂懂的，不过这题好像要知道

谢谢ranran提示“地球每年绕太阳一圈” 脑子里有这个概念后，那个加亮的句子偶又读了一遍发现读懂了，第二句是我当时理解错...

第一句是说地球经过中空部分需要一天

第二句是说，因为每次经过流星不一定正好经过流星轨道，所以每一年经过流星所用的时间和下一年会有不同，（好像在暗示地球每年经过流星一次）

然后结合第三段第一句和第六段第一句话（就是OG解释）可以选出答案。

看来做错原因还是阅读理解不过关...