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.

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.

答案选a，晕，这不是加强么．．．我选了c，请高人指点！！