在CD和google上搜了半天也没搜到OG 12，116题的明确答案，大家能不能帮帮我啊？我真的想了一个晚上都没明白啊。。哭泣。。。

dreamistrue · 发表于 2012-4-20 12:29:11

OG12 116

After evidence was obtained in the 1920s that
the universe is expanding, it became reasonable
to ask: Will the universe continue to expand
indefinitely, or is there enough mass in it for the
mutual attraction of its constituents to bring this
expansion to a halt? It can be calculated that
the critical density of matter needed to brake the
expansion and “close” the universe is equivalent
to three hydrogen atoms per cubic meter. But the
density of the observable universe—luminous matter
in the form of galaxies—comes to only a fraction
of this. If the expansion of the universe is to stop,
there must be enough invisible matter in the
universe to exceed the luminous matter in density
by a factor of roughly 70.
Our contribution to the search for this “missing
matter” has been to study the rotational velocity
of galaxies at various distances from their center
of rotation. It has been known for some time that
outside the bright nucleus of a typical spiral galaxy
luminosity falls off rapidly with distance from the
center. If luminosity were a true indicator of mass,
most of the mass would be concentrated toward
the center. Outside the nucleus the rotational
velocity would decrease geometrically with distance
from the center, in conformity with Kepler’s law.
Instead we have found that the rotational velocity
in spiral galaxies either remains constant with
increasing distance from the center or increases
slightly. This unexpected result indicates that the
falloff in luminous mass with distance from the
center is balanced by an increase in nonluminous
mass.
Our findings suggest that as much as 90
percent of the mass of the universe is not radiating
at any wavelength with enough intensity to be
detected on the Earth. Such dark matter could be
in the form of extremely dim stars of low mass,
of large planets like Jupiter, or of black holes,
either small or massive. While it has not yet been
determined whether this mass is sufficient to
close the universe, some physicists consider it
significant that estimates are converging on the
critical value.

the author`s study indicates that, in comparison with the outermost regions of a typical spiral galaxy, the region just outside the nucleus can be characterized as having
A. higher rotational velocity and higher luminosity
B. lower rotational velocity and higher luminosity
C. lower rotational velocity and lower luminosity
D. similar rotational velocity and higher luminosity
E. similar rotational velocity and similar luminosity

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答案是D，请问这题为什么不能选B？在网上搜了好久也没搜到为什么不能选B的原因。原文说的是Instead we have found that the rotational velocity in spiral galaxies either remains constant with
increasing distance from the center or increases slightly.

选D的话就是用的constant这个定位点。但是，如果用increases slightly这个定位点，就可以选B了啊。因为翻译过来是：越往外会有些许的增加，那么just outside就会比the outermost 低啊，就是Lower啊。难道这道题BD都可以吗？文章结构明明就是either or啊，为啥就选D而不选B呢？说不过去呀。有木有人帮帮我啊

Elroy · 发表于 2012-4-20 22:21:53

呵呵你选项没看清。用 increases slightly 定位的话，也应该是higher rotational velocity ,怎么能使B里面的Lower呢

猪王山 · 发表于 2012-4-20 22:47:07

slightly就是轻微的，可以理解为增长，但是忽略不计的程度。。。
综合意思再看看，上面说根据开普勒定理应该下降，但是（instead是转折，转折），保持不变或者轻微增长

dreamistrue · 发表于 2012-4-20 23:51:58

呵呵你选项没看清。用 increases slightly 定位的话，也应该是higher rotational velocity ,怎么能使B里面的Lower呢

-- by 会员 Elroy (2012/4/20 22:21:53)

是lower哦，是越往远越increases slightly ，所以just outside比the outermost 低。

在CD和google上搜了半天也没搜到OG 12，116题的明确答案，大家能不能帮帮我啊？我真的想了一个晚上都没明白啊。。哭泣。。。

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