没有找到这篇的讨论，但好像前人也有疑问。Q25 老管给的答案是B。个人认为应该是C。
Q23 to Q26:
Diamonds are almost impos-
sible to detect directly because they
are so rare: very rich kimberlite
Line pipes, the routes through which
(5) diamonds rise, may contain only
three carats of diamonds per ton
of kimberlite. Kimberlite begins as
magma in Earth’s mantle (the layer
between the crust and the core). As
(10) the magma smashes through layers
of rock, it rips out debris, creating
a mix of liquid and solid material.
Some of the solid material it brings
up may come from a so-called
(15) diamond-stability field, where condi-
tions of pressure and temperature
are conducive to the formation of
diamonds. If diamonds are to sur-
vive, though, they must shoot toward
(20) Earth’s surface quickly. Otherwise,
they revert to graphite or burn.
Explorers seeking diamonds look
for specks of “indicator minerals”
peculiar to the mantle but carried up
(25) in greater quantities than diamonds
and eroded out of kimberlite pipes
into the surrounding land. The stan-
dard ones are garnets, chromites,
and ilmenites. One can spend years
(30) searching for indicators and tracing
them back to the pipes that are their
source; however, 90 percent of
kimberlite pipes found this way are
barren of diamonds, and the rest
(35) are usually too sparse to mine.
In the 1970’s the process of
locating profitable pipes was refined
by focusing on the subtle differ-
ences between the chemical
(40) signatures of indicator minerals
found in diamond-rich pipes as
opposed to those found in barren
pipes. For example, G10 garnets,
a type of garnet typically found in
(45) diamond-rich pipes, are lower in
calcium and higher in chrome than
garnets from barren pipes. Geo-
chemists John Gurney showed that
garnets with this composition were
(50) formed only in the diamond-stability
field; more commonly found ver-
sions came from elsewhere in the
mantle. Gurney also found that
though ilmenites did not form in the
(55) diamond-stability field, there was a
link useful for prospectors: when
the iron in ilmenite was highly
oxidized, its source pipe rarely
contained any diamonds. He rea-
(60) soned that iron took on more or less
oxygen in response to conditions in
the kimberlitic magma itself—mainly
in response to heat and the avail-
able oxygen. When iron became
(65) highly oxidized, so did diamonds;
that is, they vaporized into carbon
dioxide.
Q25:
The passage suggests that the presence of G10 garnet in a kimberlite pipe indicates that
the pipe in which the garnet is found has a 90% chance of containing diamonds
the levels of calcium and chrome in the pipe are conducive to diamond formation
the pipe passed through a diamond-stability field and thus may contain diamonds
any diamonds the pipe contains would not have come from the diamond-stability field
the pipe’s temperature was so high that it oxidized any diamonds the pipe might have contained | 
