GRE阅读练习每日一篇(十八)_GRE阅读

　　17. According to the passage, which of the following is a direct perceptual consequence of interstellar dust?

　　Some stars are rendered invisible to observers on Earth.

　　Many visible stars are made to seem brighter than they really are.

　　The presence of hydrogen and helium gas is revealed.

　　The night sky appears dusty at all times to observers on Earth.

　　The dust is conspicuously visible against a background of bright stars.

　　18. It can be inferred from the passage that the density of interstellar material is

　　higher where distances between the stars are shorter

　　equal to that of interstellar dust

　　unusually low in the vicinity of our Sun

　　independent of the incidence of gaseous components

　　not homogeneous throughout interstellar space

　　19. It can be inferred from the passage that it is because space is so vast that

　　little of the interstellar material in it seems substantial

　　normal units of volume seem futile for measurements of density

　　stars can be far enough from Earth to be obscured even by very sparsely distributed matter

　　interstellar gases can, for all practical purposes, be regarded as transparent

　　optical astronomy would be of little use even if no interstellar dust existed

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