【正确答案】 B

【答案解析】Supporting ideas
This question requires recognizing information that is provided in the passage. The first paragraph describes and compares two types of galaxies: conventional galaxies and dim, or low-surface-brightness, galaxies. It states that dim galaxies have the same approximate number of stars as a common type of conventional galaxy but tend to be larger and more diffuse because their mass is spread over wider areas (lines 4-10). The passage also indicates that dim galaxies take longer than conventional galaxies to convert their primordial gases into stars, meaning that dim galaxies evolve much more slowly than conventional galaxies (lines 10-14), which entails that conventional galaxies evolve more quickly than dim galaxies.
A The passage states that dim galaxies have approximately the same numbers of stars as a common type of conventional galaxy.
B Correct. The passage indicates that dim galaxies evolve much more slowly than conventional galaxies, which entails that conventional galaxies evolve more quickly.
C The passage states that dim galaxies are more spread out, and therefore more diffuse, than conventional galaxies.
D The passage does not mention the relative amounts of helium in the two types of galaxies under discussion.
E The passage states that dim galaxies tend to be much larger than conventional galaxies.
The correct answer is B.

【正确答案】 C

【答案解析】Inference
This question requires drawing an inference from information given in the passage. The first paragraph compares dim galaxies and conventional galaxies. Dim galaxies are described as having the same general shape (lines 4-5) as a common type of conventional galaxy, the spiral galaxy, suggesting that dim galaxies are, themselves, spiral shaped. The passage also indicates that, although both types of galaxies tend to have approximately the same number of stars, dim galaxies tend to be much larger and spread out over larger areas of space (lines 4-10) than conventional galaxies.
A The passage states that the two types of galaxies have approximately the same number of stars.
B The passage indicates that dim galaxies are relatively large and spread out.
C Correct. The passage indicates that dim galaxies have the same general shape as spiral galaxies and that their mass is spread out over large areas of space.
D The passage indicates that dim galaxies are relatively large and spread out.
E The passage states that dim galaxies have few stars per unit of volume, suggesting that they are not dense but diffuse.
The correct answer is C.

【正确答案】 B

【答案解析】Inference
This question requires drawing an inference from information given in the passage. The second paragraph describes the long-standing puzzle of the missing baryonic mass in the universe. The passage states that baryons are the source of galactic luminosity, and so scientists can estimate the amount of baryonic mass in the universe by measuring the luminosity of galaxies (lines 17-21). The puzzle is that spectroscopic measures of helium in the universe suggest that the baryonic mass in the universe is much higher than measures of luminosity would indicate (21-25).
A The differences between the rates of evolution of the two types of galaxies is not treated as being controversial in the passage.
B Correct. The passage indicates that measurements using spectroscopy and measurements using luminosity result in puzzling differences in estimates of the universe's baryonic mass.
C The passage does not suggest how helium might relate to the numbers of stars in dim galaxies.
D The passage indicates that astronomers have speculated that the missing baryonic mass might be discovered in intergalactic space or hard-to-detect galaxies but does not suggest that these speculations are constituents of the long-standing puzzle.
E The passage does not mention how the distance to galaxies affects scientists' ability to detect these galaxies.
The correct answer is B.

【正确答案】 A

【答案解析】Inference
This question requires drawing an inference from information given in the passage. The puzzle is that estimates of the baryonic mass of the universe based on luminosity are lower than those based on spectroscopy (lines 21-25).The passage states that astronomers did not notice dim galaxies until recently (lines 2-3) and that these galaxies may help account for the missing baryonic mass in the universe (lines 15-17).The passage also suggests that astronomers measure the luminosity of specific galaxies (lines 19-21). Thus it can be inferred that, prior to their being noticed by astronomers, the luminosity of these dim galaxies was not measured, and their baryonic mass was not taken into account in the estimates of luminosity that led to the longstanding puzzle.
A Correct. The passage states that the missing baryonic mass in the universe may be discovered in the dim galaxies that have only recently been noticed by astronomers.
B The passage does not suggest that dim and conventional galaxies both originating from primordial gas clouds help solve the long-standing puzzle of the missing baryonic mass in the universe.
C The passage does not suggest that dim galaxies might contain more helium than do conventional galaxies or that measures of baryonic mass using spectroscopy do not take some dim galaxies into account.
D The passage does not suggest that dim galaxies contain more baryonic mass than scientists originally believed upon discovering these galaxies.
E The passage suggests that scientists measured the luminosity of galaxies, not of individual stars.
The correct answer is A.

【正确答案】 C

【答案解析】Evaluation
This question requires understanding how one aspect of the passage relates to the reasoning in a larger portion of the passage. The second paragraph explains that scientists have been puzzled over missing baryonic mass in the universe as measured by luminosity (lines 21-25). Given that baryons are the source of luminosity in the galaxy (lines 17-19), astronomers can estimate the baryonic mass of a galaxy by measuring its luminosity.
A The passage discussion of baryons does not address the number of stars in individual galaxies.
B The passage discusses how the luminosity of galaxies can be used to estimate baryonic mass but does not address how total luminosity is measured.
C Correct. The passage indicates that because baryons are the source of galactic luminosity, measuring luminosity can be used to estimate baryonic mass of galaxies.
D The passage suggests that estimates based on luminosity may have been less accurate, not more accurate, than those based on spectroscopy.
E The passage does not indicate that bright galaxies contain more baryons than do dim galaxies.
The correct answer is C.

【正确答案】 E

【答案解析】Inference
This question involves identifying which answer option potentially conflicts with the information the author has provided in the passage. The second paragraph indicates that astronomers' estimates of the baryonic mass of the universe is lower when measured using luminosity than it is when measured using spectroscopy (lines 21-25). The final sentence states that astronomers have speculated that the missing baryonic mass might be discovered in intergalactic space or in hard-to-detect galaxies (lines 25-29). Although the passage does indicate that the discovery of dim, low-surface-brightness galaxies might help account for the missing baryonic mass (lines 15-17), the passage provides no support for the possibility that baryonic mass has been discovered in intergalactic space.
A The passage indicates that low-surface-brightness galaxies went unnoticed until recently, unlike conventional galaxies.
B The passage indicates that low-surface-brightness galaxies have the same general shape as spiral galaxies.
C The passage describes two possible explanations astronomers have given for the missing baryonic mass, one of which was made more plausible by the discovery of low-surface-brightness galaxies.
D The passage indicates that astronomers have used spectroscopy to estimate baryonic mass and gives no reason to suspect that this method is not useful.
E Correct. The passage does not indicate that astronomers have found any baryonic mass in intergalactic space.
The correct answer is E.

【正确答案】 A

【答案解析】Main idea
This question requires understanding, in broad terms, the purpose of the passage as a whole. The first paragraph describes a phenomenon: the discovery of dim galaxies and some of their general attributes. The second paragraph describes how this discovery may help astronomers to solve a long-standing puzzle about the baryonic mass of the universe.
A Correct. The passage describes the phenomenon of dim galaxies and describes their significance in solving the longstanding puzzle of the missing baryonic mass in the universe.
B Although the passage discusses the puzzling difference between the two estimates of baryonic mass, this option does not account for the broader topic of dim galaxies.
C While the passage identifies the newly discovered phenomenon of dim galaxies, it does not offer a significant explanation for these galaxies' origins.
D Although the passage compares dim an 1 conventional galaxies in the first paragraph, this option does not account for the important detail that dim galaxies may help solve a long-standing puzzle.
E The discovery of dim galaxies discussed in the passage is not said to be inconsistent with any existing scientific theory.
The correct answer is A.