Q36:

The author mentions the fact that baryons are the source of stars’ luminosity primarily in order to explain

1. how astronomers determine that some galaxies contain fewer stars per unit volume than do others
2. how astronomers are able to calculate the total luminosity of a galaxy
3. why astronomers can use galactic luminosity to estimate baryonic mass
4. why astronomers’ estimates of baryonic mass based on galactic luminosity are more reliable than those based on spectroscopic studies of helium
5. 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.

你划线的这句话正是为了说明前句话，前句话提到了Baryron的质量不见了，后面接着讲Baryon的数量是在galaxy luminosity的基础上估计的，这个数量是和质量相关的.

這題我覺得C也有點問題。因為，原文說的是由galaxy的亮度來估計bryons的「數量」（L34-35 their numbers can be estimated based on how luminous galaxies are），但是c答案卻是說太空人可利用亮度來估計baryonic mass(質量)。

因此，我覺得，答案應該是E。

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. 對應下面 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. 所以是解釋為什麼，而不是how 小小淺見...望NN指點...

many thanks !!!!!

有人能分析一下这句话的语法结构吗？