下面进行GMAT考试重点试题整理，希望考生充分利用文中所列内容，提高备考效率。
27. GWD-27-Q10-Q12 黑洞存在的研究
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.
以上是对GMAT考试重点试题整理的介绍，希望对考生有所启发，能够加快备考进度，取得GMAT高分。
