2. 外语
  3. This passage is excerpted from material published in 1997. Is there a massive black hole at the center of our galaxy, the Mi

This passage is excerpted from material published in 1997. Is there a massive black hole at the center of our galaxy, the Mi

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问题 This passage is excerpted from material published in 1997.
    Is there a massive black hole at the center of our galaxy, the Milky Way? The evidence is inconclusive. Just as the Sun’s mass can be determined, given knowledge of other variables, by the velocity at which its planets orbit, the mass at the center of the Milky Way can be revealed by the velocities of stars and gas orbiting the galactic center. This dynamical evidence, based on recently confirmed assumptions about the stars’ velocities, argues for an extremely compact object with a mass two to three million times the mass of our Sun. Although according to current theory this makes the mass at the center of the galaxy too dense to be anything but a black hole, the relative lack of energy radiating from the galactic center presents a serious problem. A black hole’s gravity attracts surrounding matter, which swirls around the black hole, emitting some energy as it is engulfed. Scientists believe that the amount of energy that escapes the black hole should be about 10 percent of the matter’s rest energy (the energy equivalent of its mass according to the equation E=mc2). But when the energy coming from the galactic center is compared to widely held predictions based on how much matter should be falling into a theoretical central black hole, there is a discrepancy by a factor of a few thousand.  
The primary purpose of the passage is to
选项 A、present several theories that could account for a particular phenomenon
B、argue that a certain question needs to be reframed in light of new evidence
C、resolve an apparent inconsistency between two lines of evidence
D、explain why a certain issue remains unresolved
E、present evidence that calls into question certain assumptions of a current theory
答案D
解析 Main idea
Our goal is to determine which of the five options best expresses the primary purpose of the passage. Note that this question regards the topic and how that topic is discussed: the issue that the passage primarily focuses on concerns the mass at the center of our galaxy. The passage primarily aims to explain why the specific nature of that mass had not—at least in 1997, when the passage was written—been adequately understood.
A     While the passage draws upon certain theoretical findings, it does not present multiple theories.
B     The passage suggests that the author is puzzled by the enormous mass at the center of the Milky Way, in light of evidence showing the relatively low level of energy radiating outward. Nevertheless, the passage never attempts to convince us that the question needs to be reframed.
C     The passage describes an apparent inconsistency—or discrepancy—between two lines of evidence but does not seek to resolve that inconsistency.
D     Correct. As indicated above, this choice expresses the primary purpose of the passage; that is, to explain why the nature of the mass at the center of our galaxy was not adequately understood.
E     Although some of the evidence presented may cast doubt on one or more assumptions of a theory that was current when the passage was written, the passage itself is not primarily focused on identifying any such assumptions.
The correct answer is D.
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