Reading Comprehension

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 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 shows 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.

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 for most of the gravitational perturbation affecting the Geminid meteor stream.

Q/A-C can someone explain the answer.

Hello!

This is an inference question, so we should take a small "step" from what the author actually says in the passage to our conclusions based on these words. These conclusions should be quite obvious, because if they are not, then you're probably taking a "step" too large to be the correct answer. When stuck between two choices, I'd advise picking the answer with a smaller "step" from the author's actual words to conclusions based on these words.

However, at first we should find these actual words in the text: "the time interval between the two bursts of activity would vary from one year to the next". From this we can conclude that if it varies from one year to the next, the crossing occurs once each year. If if were otherwise, for example, once a month, then the text would say, "from one month to the next", because earlier the passage stated, "there is no reason why the Earth should always pass through the stream's exact center" (and we can conclude that the time during which the Earth passes the stream varies each time).

Also, the sentence "The Geminid data between 1970 and 1979 ... an average of 19 hours ..." slightly backs up our conclusion (it shows that the Earth crossed the stream several times during that period).

Hi,

A. No where in the passage is it mentioned that "most" i.e. >50% of the time, the meteor stream that the Earth encounters are more than 2,000 years old. Not justified ->Incorrect.
B. In fact, this is a 180 degree answer. The exact oposite is stated in the end of the 2nd paragraph" There is no reason why the Earth should always pass through the stream's exact center,". Incorrect.
C. "from one year to the next" ; "actual yearly Geminid meteor shower" -->These phrases imply that the earth must pass through it every year exactly once. Correct.
D. "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.". Only IF the Geminid stream were 5,000 years old, this answer choice would hold true. But that has not been established beyond reasonable doubt. Incorrect.
E. The passage does mention that the earth contributes to "most" of the gravitational disturbances and hence cannot be concluded. Incorrect.