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 pa rent comet, but due to their differing 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 experiment 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. Conventional
theories, however, predicted that the distribution of particles would be
increasingly dense toward the center of a meteor stream. Surprisingly, 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 1000 years old.
56. The primary focus of the passage is on which of the
following?
(A) Comparing two
scientific theories and contrasting the predictions that each would make
concerning a natural phenomenon
(B) Describing a new
theoretical model and noting that it explains the nature of observations made
of a particular natural phenomenon
(C) Evaluating the
results of a particular scientific experiment and suggesting further areas for
research
(D) Explaining
how two different natural phenomena are related and demonstrating a way to
measure them
(E) Analyzing
recent data derived from observations of an actual phenomenon and constructing
a model to explain the data
A跟B都好像對.
OG說A不對因為the passage並未有"contrast the predictions of competing theories". 可是paragraph #1的最後一句不是在比較competing theories(conventional theories and the theories dervied from the computer model)的predictions嗎?
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发表于 2008-5-14 11:03:00
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