| 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.
1,如何翻译这句话？
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.
是否可以理解为：
没有理由知道为什么（地球总是经过星河确切的中心却每年穿越星河的时间间隔是不同的）。
即：虽然每年地球都要经过星河的中心，但穿越星河的时间每年是不同的
2，87题，虽然可以理解E是正确答案，但如何解释D?
87. Which of the following is an assumption underlying the last sentence of the passage?
(A) In each of the years between 1970 and 1979, the Earth took exactly 19 hours to cross the Geminid meteor stream.
(B) The comet associated with the Geminid meteor stream has totally disintegrated.
(C) The Geminid meteor stream should continue to exist for at least 5,000 years.
(D) The Geminid meteor stream has not broadened as rapidly as the conventiona ltheories would have predicted.
(E) The computer-model Geminid meteor stream provides an accurate representation of the development of the actual Geminid stream.
另外，又如何理解OG对D的解释？
because the computer model is said to confirm the broadening predicted by the conventional theories; the fact that the model projected the positions of the particles in the stream over a 5000year period does not suggest that researchers expected the stream to be older and therefore broader than it turned out to be.
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