| 这个文章做的不好， 看了总结里的讨论
第84题目仍然不是很明白
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(碎裂瓦解) comet until a shroud (覆盖)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. (上面的hypothesis就是传统理论)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(86题)? 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.
84：
84. According to the passage, why do the dust particles in a meteor stream eventually surround a comet’s orginla orbit?
(A) They are ejected by the comet at differing velocities.
(B) Their orbits are uncontrolled（错误，应该是被万有引力控制） by planetary gravitational fields.
(C) They become part of the meteor stream at different times.
(D) Their velocity slows over time.
(E) Their ejection velocity is slower than that of the comet.
This question asks for the reason given in the passage for a characteristic feature of meteor
streams. According to lines 1-7, the dust particles in a meteor stream eventually surround a
comet’s original orbit because of the different velocities at which they are ejected, as stated in
choice A, the best answer.
Choice B is directly contradicted by information in the passage (lines 8-10). The other answer
choices re incorrect because the passage does not say that the dust particles become part of the
meteor stream at different times, or that their velocity slows over time, or that their ejection
velocity is slower than that of the comet.
虽然定位在line1-7，可是看不出，particle绕那个comet旋转，是由于速度不同造成的啊
请牛人指点
我真没看明白
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