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  3. The primary focus of the passage is on which of the following? The passage suggests that which of the following is a prediction

The primary focus of the passage is on which of the following? The passage suggests that which of the following is a prediction

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问题 The primary focus of the passage is on which of the following?
The passage suggests that which of the following is a prediction concerning meteor streams that can be derived from both the conventional theories mentioned in line 18 and the new computer-derived theory?
选项 A、Dust particles in a meteor stream will usually be distributed evenly throughout any cross section of the steam.
B、The orbits of most meteor streams should cross the orbit of the Earth at some point and give rise to a meteor shower.
C、Over time the distribution of dust in a meteor stream will usually become denser at the outside edges of the stream than at the center.
D、Meteor showers caused by older by older meteor streams should be, on average, longer in duration than those caused by very young meteor streams.
E、The individual dust particles in older meteor streams should be, on average, smaller than those that compose younger meteor streams.
答案D
解析 This question asks you to identify a prediction that can be derived from both the conventional theories about meteor streams and the new computer-derived model. You must base your answer on information that is suggested by, but not expressly stated in, the passage. According to lines 7-8 of the passage, the conventional theories hypothesized that meteor streams should broaden with time, and the computer simulation confirmed this hypothesis. The passage also suggests that the time it takes for the Earth to cross a meteor stream (and, by implication, the duration of the resulting meteor shower) is directly related to the breadth of the stream (lines 23-28). From these pieces of information, which are supported by both the conventional theories and the new computer-derived theory, it can be inferred that on average the meteor showers caused by older (and therefore broader) meteor streams would be longer in duration than those caused by very young (and therefore narrower) meteor streams, as stated in D, the best answer. Choice A is incorrect because it contradicts the predictions of both the conventional theories (that the particles will be most dense at the center of the stream) and the computer model (that the stream will resemble a thick-walled, hollow pipe). Choice C is also incorrect because it is inconsistent with the conventional theories that suggested the distribution of dust in a meteor stream is denser at the center. And choices B and E are incorrect because the theories discussed in the passage do not suggest anything about the likelihood that the Earth’s orbit will cross that of any particular meteor stream, nor do they suggest anything about the size of the dust particles that compose meteor streams.
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