I think the answer is c but I looked it up and got this: system or thing used as an example to follow or imitate.
The area around the rock will have weathered down and the rock will eventually either break or it will fall down the mountain
This process is called Denitrification. The process is undertaken by the denitrifying bacteria, whose action is converting nitrates in the soil to free atmospheric nitrogen and therefore, depleting soil fertility and reducing agricultural productivity. Examples of denitrifying bacteria are thiobacillus denitrificans, micrococcus denitrificans among others.