黑洞存在的研究GWD-27-Q12

2006gmat2006

 Q12:

Which of the following, if true, would most clearly undermine the possible explanation for the whirling gas in M87 that is mentioned in the last sentence of the first paragraph?

                       

1. The stars in a star cluster at the center of M87 could exert a strong gravitational force without tearing the cluster apart.
2. A cluster of stars at the center would preclude the existence of certain other astronomical phenomena that have been observed at the center of M87.
3. The stars within many existing galaxies, such as NGC 4258, are more closely spaced than are the stars within the core of M87.
4. Only one other galaxy has been observed to contain gas clouds whirling about its center as they do about the core of M87.
5. The gravitational force of a cluster of a billion or so dim stars would be sufficient to cause a whirling ring of gas and dust to collect around the center of a galaxy.

答案是A  ???????

2006gmat2006

Astronomers theorize that a black

            hole forms when a massive object

            shrinks catastrophically under its own

Line gravity, leaving only a gravitational

  (5)field so strong that nothing escapes it.

Astronomers must infer the existence

of black holes, which are invisible,

from their gravitational influence on

the visible bodies surrounding them.

 (10)For example, observations indicate

that gas clouds in galaxy M87 are

whirling unusually fast about the gal-

axy’s center.  Most astronomers

believe that the large concentration

 (15)of mass at the galaxy’s center is a

black hole whose gravity is causing

the gas to whirl.  A few skeptics have

argued that the concentration of mass

necessary to explain the speed of the

 (20)whirling gas is not necessarily a black

             hole:  the concentration in M87 might

             be a cluster of a billion or so dim stars.

  The same hypothesis might have

been applied to the galaxy NGC 4258,

 (25)but the notion of such a cluster’s

existing in NGC 4258 was severely

undermined when astronomers mea-

sured the speed of a ring of dust and

gas rotating close to the galaxy’s

 (30)center.  From its speed, they calcu-

lated that the core’s density is more

than 40 times the density estimated

            for any other galaxy.  If the center of

NGC 4258 were a star cluster, the

 (35)stars would be so closely spaced

that collisions between individual

stars would have long ago torn the

cluster apart.

 

trilliamz

能解释一下A 吗?

我的选择是B

题目说要找可以"undermine" the explanation mentioned in "the concentration in M87 might be a cluster of a billion or so dim stars"

作者在第二段末反驳这个假设 因为: "from its speed, they calculated that the core's density is ore than 40 tims the density...so closely spaced that collisions between individual stars would have long ago torn the cluster apart."

所以说 A 加强了解释.