gwd-5-25 为什么不选c而选b呀

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.

A.   the pipe in which the garnet is found has a 90% chance of containing diamonds

B.    the levels of calcium and chrome in the pipe are conducive to diamond formation

C.   the pipe passed through a diamond-stability field and thus may contain diamonds

D.   any diamonds the pipe contains would not have come from the diamond-stability field

E.    the pipe’s temperature was so high that it oxidized any diamonds the pipe might have contained

indicate的题目还是看上下文以及作者想借这个例子说什么

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. 

这一段就是说通过分析化学成分的不同确定贫矿富矿，那么G10这种pipe含有很多钙，同时又是富矿，不就说明了conducive吗？有点像逻辑的架桥

十分感谢，茅塞顿开

这道题答案到底是B还是C啊，我觉得应该选C

原文说，G10 "a type of garnet typically found in diamond-rich pipes... this composition were formed only in the diamond-stability field" 

选项C正是原文的精确改写

而B 说的conducive to diamond formation，即有助于钻石的形成，而文章的意思是这种level 有助于判断贫矿富矿，即通俗点说就是是不是含钻石， 是完全不同的两个概念。

只是自己的一些看法，还请NN赐教～

我也觉得选C哎，答案怎么是B呢