Q36:
The author mentions the fact that baryons are the source of stars’ luminosity primarily in order to explain
how astronomers determine that some galaxies contain fewer stars per unit volume than do others
how astronomers are able to calculate the total luminosity of a galaxy
why astronomers can use galactic luminosity to estimate baryonic mass
why astronomers’ estimates of baryonic mass based on galactic luminosity are more reliable than those based on spectroscopic studies of helium
how astronomers know bright galaxies contain more baryons than do dim galaxies
答案是C，但我觉得怎么是B呀。。请看原文中内容：
Baryons—subatomic particles
(30) that are generally protons or
neutrons—are the source of
stellar, and therefore galactic,
luminosity, and so their numbers
can be estimated based on how
(35) luminous galaxies are.
这里是说Baryons是stellar的source，因此通过galaxies的亮度，可以估算出单个galaxy的亮度，因此也就可估算出galaxy的数目。而原文中的therefore正是和题目中的in order to相对应的，答案B中说的正是“单个galaxy的发光度”，而C中说的galaxy的质量,在此处没有提及，是后文推出了。所以,应该选C呀。。请大家帮忙看看，非常感谢。。
原文附下：
In addition to conventional
galaxies, the universe contains
very dim galaxies that until
Line recently went unnoticed by
(5) astronomers. Possibly as
numerous as conventional gal-
axies, these galaxies have the
same general shape and even
the same approximate number
(10) of stars as a common type of
conventional galaxy, the spiral,
but tend to be much larger.
Because these galaxies’
mass is spread out over
(15) larger areas, they have far
fewer stars per unit volume
than do conventional galaxies.
Apparently these low-surface-
brightness galaxies, as they
(20) are called, take much longer
than conventional galaxies to
condense their primordial gas
and convert it to stars—that is,
they evolve much more slowly.
(25) These galaxies may
constitute an answer to the long-
standing puzzle of the missing
baryonic mass in the universe.
Baryons—subatomic particles
(30) that are generally protons or
neutrons—are the source of
stellar, and therefore galactic,
luminosity, and so their numbers
can be estimated based on how
(35) luminous galaxies are. How-
ever, the amount of helium
in the universe, as measured
by spectroscopy, suggests
that there are far more baryons
(40) in the universe than estimates
based on galactic luminosity
indicate. Astronomers have
long speculated that the missing
baryonic mass might eventually
(45) be discovered in intergalactic
space or as some large popu-
lation of galaxies that are difficult
to detect. | 
