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 answer may lie in :

(50) formed only in the diamond-stability

field; more commonly found ver-

sions came from elsewhere in the

mantle.

B is right.