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.



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.


Can anybody explain why C is the correct answer not B.