answer is A. Pls refer to the following passage for details: 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. | 
