求助!og10-14-63

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 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.

83. It can be inferred from the passage that which of the following would most probably be observed during the Earth's passage through a meteor stream if the conventional theories mentioned in line 18 were

A. Meteor activity would gradually increase to a single, intense peak, and then gradually decline.

B. Meteor activity would be steady throughout the period of the meteor shower.

C.Meteor activity would rise to a peak at the beginning and atthe end of the meteor shower.

D. Random bursts of very high meteor activity would be interspersed with periods of very little activity.

E. In years in which the Earth passed through only the outer areas of a meteor stream, meteor activity would be absent.

答案A前半部分能找出来,"and then gradually decline."怎么推出来的?不明白....请各位xdjm帮忙.最好详细解答一下这道题的在文中怎么推出来的........谢谢~~~~~

83.

This question asks you to make an inference about what would most probably be observed during

the Earth’s passage through a meteor stream if the conventional theories mentioned in the passage

were correct. According to line 18-20, the conventional theories predicted that the meteor stream

would be most dense at the center. The computer model, one the other hand, predicted that a

meteor stream would come to resemble a thick-walled, hollow pipe (lines 21-22). The passage

states that, if the computer model were correct, two peak periods of meteor activity would be

observed as the Earth passed through the walls of the “pipe” (lines 28-31). According to lines

36-38, observational data confirmed the prediction of the computer model. If, on the other hand,

the conventional theories were correct, it can be inferred that a bifurcation of meteor activity

would not be observed; instead, it can be inferred that scientists would expect to observe a single

peak of meteor activity as the Earth passed through the dense center of the stream. Choice A

identifies this single peak of activity as the most likely observation if the conventional theories

were correct.

Choice B and D are not correct because they describe meteor activity that is either steady or

erratic, neither of which is consistent with the conventional theories. Choice C describes meteor

activity more in line with the bifurcation predicted by the computer model, rather than the single

peak of activity that the conventional theories would suggest. Choice E is incorrect because the

passage says that meteor showers occur whenever the Earth passes through a meteor stream; it

cannot be inferred that either theory would predict otherwise.