Answer: Option C
Answer: (c)
From the depth of burial by continual sediment deposition from above, or from compressional stress from tectonic plate collisions, shale is metamorphosed into slate over periods of millions of years.
During this compression, the clay minerals making up the shale decompose as they become unstable in the high-pressure environment, and their chemical components are gradually transformed into minerals that are more stable in the newly forming higher pressure environment.