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  3. This passage is excerpted from material published in 1997. Is there a massiv

This passage is excerpted from material published in 1997. Is there a massiv

游客2024-01-12  78
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.  [br] The "serious problem" referred to in line 17 could be solved if which of the following were true?
选项 A、Current assumptions about how much matter a black hole would engulf proved to be several thousand times too high.
B、Current assumptions about how much matter a black hole would engulf proved to be a few thousand times too low.
C、The object at the center of the Milky Way turned out to be far more dense than it is currently estimated to be.
D、The object at the center of the Milky Way turned out to be far more massive than it is currently estimated to be.
E、Matter being engulfed by a black hole radiated far more energy than is currently assumed.
答案 A
解析 Which one of the five options would, if true, indicate a possible solution to the serious problem referred to? The serious problem is said to arise from the relative lack of energy radiating from the galactic center compared with the expected value if a
matter-engulfing black hole were truly at the center of the Milky Way.
According to the passage, the radiated energy turned out to be a few thousand times less than had been expected, on theoretical grounds, to radiate from mass engulfed by the hypothetical black hole. However, if it were discovered that the matter engulfed were several thousand times less than previously estimated, the relatively low level of radiated energy observed would no longer seem at odds with the existence of the hypothesized black hole.
A      Correct. Reviewing the assumptions underlying the widely held predictions could lead to revised predictions that harmonize with the observational evidence regarding radiated energy from the galaxy’s center.
B      The passage states that the assumed quantity of engulfed matter already seems to be radiating too little; this answer choice would actually make the "serious problem" in the passage worse. That is, assuming that there is actually more engulfed matter with such a small amount of radiation would simply worsen the problem.
C      The passage does not specify what impact a greater-than-estimated density would have on the quantity of energy that is radiated. That is, even if greater mass would change predictions, greater density may not do so.
D     If the object were more massive, then presumably the matter engulfed by this hypothetical more massive black hole would radiate a quantity of energy even greater than the observed quantity. This again would simply make the "serious problem" worse.
E      Given that not enough radiated energy is currently being observed, finding that the matter being engulfed radiates even more energy than expected would make the "serious problem" significantly worse. That is, the relatively low quantity of radiated energy observed would fall even farther short of the quantity expected.
The correct answer is A.
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