困惑：GWD习题与XDF教材中同题答案不同，请教G友！

wongmichelmm2

GWD-TN-4中第三篇阅读和XDF蓝皮教材133页的PASSAGE48同题，具体如下：
     Diamonds are almost impossible to detect directly because they are so rare: very rich kimberlite pipes, the routes through which 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 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 diamond-stability field, where conditions of pressure and temperature are conducive to the formation of diamonds.  If diamonds are to survive, though, they must shoot toward 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 in greater quantities than diamonds and eroded out of kimberlite pipes into the surrounding land.  The standard ones are garnets, chromites, and ilmenites.  One can spend years 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 are usually too sparse to mine.
     In the 1970’s the process of locating profitable pipes was refined by focusing on the subtle differences between the chemical  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 diamond-rich pipes, are lower in calcium and higher in chrome than garnets from barren pipes.  Geochemists John Gurney showed that garnets with this composition were formed only in the diamond-stability field; more commonly found versions came from elsewhere in the mantle.  Gurney also found that though ilmenites did not form in the 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 reasoned that iron took on more or less oxygen in response to conditions in the kimberlitic magma itself—mainly in response to heat and the available oxygen.  When iron became highly oxidized, so did diamonds; that is, they vaporized into carbon dioxide.
GWD6-Q25:
The passage suggests that the presence of G10 garnet in a kimberlite pipe indicates that
        
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

GWD选C，XDF蓝皮教材选B。

wongmichelmm2

请教各位牛人，如何解？